I’d send back a huge lineman like 6’6", 335 lb Lincoln Kennedy. Just to see the reaction from the opposing team as he stands up and blocks out the sun. Although it might be fun to see Michael Vick play in an old single wing formation. Heck, maybe the Falcons should try that…
For basketball, I would send back Yao Ming, just to see the expressions on people’s faces at the sight of a seven and a half foot tall Chinaman. 
I agree with the Barry Sanders opinion.
He would utterly unstoppable in the 50s and could probably be more of a power runner back then if he wanted to be (compared to the 90s).
I’m not so sure a modern baseball player would have much impact on the game of yore. (So long as you deny the modern player his steroids.)
Baseball used to be THE sport in this country. Almost all great athletes aspired to be baseball players. This meant that back in the day, baseball had the best athletes the country could produce. Today, a great athlete can choose from among baseball, basketball and football. (I exclude hockey for the obvious reason that no one really cares about it.) In 1955, if you had a choice between baseball and any other sport, you chose baseball.
I offer Dick Groat as exhibit A for this. A two-sport star of the era, he was forced to choose between baseball and basketball, and chose the former. Though he wound up as National League MVP, the general consensus was that basketball was his best game.
So I think a modern baseball player going back to 1955 might be surprised at just how good his competition would be.
Good thoughts so far.
I think a modern NFL lineman would just obliterate opponents from the 50’s. Someone like Jonathon Ogden who is quick and agile despite being 6’9" and 340 lbs would just destroy the 275 lb “giants” of then. Jevon Kearse would blow them away too being 265 lbs and faster than the WR’s of the 50’s. Moss would equally dominate. He’d be unstoppable (with today’s rules). Just about any modern NFL player would be physcially dominant
For baseball, any modern pitcher would dominate for a little while, until the lack of consistent relief pitching caught up to them. A modern pitcher has a much more complicated repertoire of pitches than one from 50 years ago. Too, anyone who threw a splitter would be unhittable since that pitch didn’t exist 50 years ago.
Basketball is like the NFL in that modern players are so physically superior to players from 50 years ago that it isn’t close. Fetus makes a good point about lack of fundamentals though (and the NBA is the worst for this), but I think the overall skill level would make up for it. A more important point is that the OP said modern rules go back too. A modern NBA player would foul out in 5 minutes of a 1950’s refereed game. It’d be fun to send Magic Johnson back in his prime (a 6’9" point guard???!!!), plus he had the fundamentals to play in any era.
Heh heh. Let’s pick the five biggest NFL linemen at their position and send them all back. Ogden at one tackle, Kennedy at the other. Who are some big guards and centers? The only problem is that any QB of that era wouldn’t be able to see over them. Maybe just QB sneak each time until you’re in the end zone?
Dude, Chinaman is not the preferred nomenclature. Asian-American, please. 
I’ll third Barry Sanders. He did things that you simply did not think were possible, and that was in the mid-90s.
But can we please send him back to right after the merger, so that the Lions can at least say they’ve won ONE Super Bowl?
If you subjected a number of today’s guards to traveling and carrying rules as they used to be enforced, it would probably cut down on their effectiveness. Carrying is extinct and you have to go pretty far to get called for traveling, with ‘jump-stops’ being the norm these days and so on.
Popular wisdom says otherwise, but tennis technology helps serve returners and baseliners the most. The big serves have gotten bigger, but there were guys with huge serves back then and it was very hard to deal with them using a wooden racquet. Racquets now are more forgiving and you don’t need to strike the ball perfectly to get a good shot. Pete Sampras would have stood among the greats 50 years ago, and I think Federer would have as well. They both play the classic serve-and-volley style that almost nobody uses now. Sampras had a great serve and no end of guts. He was always at his best at the key points. Federer is an artiste. He has all the shots, great touch at net, and his placement on serve is outstanding, which is why he doesn’t need to hit as hard as the big guys.
Soccer: some incredible ball handling skills at pace. Would love to see Zidane or Henry or even just Rooney go back in time.
Figure Skating: Send Michelle Kwan back fifty years. She meshes today’s athleticism with the grace that was esteemed back then. She’d blow people away.
Running: Middle distance folks and especially long distance. It’s been just over fifty years since Roger Bannister broke the four minute mile. The current record is more than 15 seconds faster. In women’s running women didn’t even run this race, but the current WR holder would just beat the men’s record holder from 1955.
Swimming wouldn’t even be close. Times in that keep dropping at an incredible rate. I’d love to send Phelps or Thorpe back to the 1956 Olympics.
For football, someone like Darryl Green or Manning is all wrong. They’d be good, but they’re just talented or fast. But, someone like Ogden or Ray Lewis who is both WAY BIGGER and WAY FASTER than guys back then would have a devastating effect on the other side. You simply could not run the ball against any team with Ray Lewis on it. You probably couldn’t pass to well either if you brought him on a pass rush.
Forget Barry Sanders. He was just quick. He wasn’t even super fast. Send back a big somebody like TJ Duckett or Mike Alstott and fahgettaboutit. They’re as heavy as old D-Lineman. They’d get 5 yards before they were touched, and run another 5 after contact every time.
Baseballers. . .I don’t know why you think RJ would be so dominant. There were some tall pitchers back then, and even (supposedly) guys who threw up around 100. RickJay knocked that one out of the park when he sent back Ichiro.
Basketballers. . .tough to say. Shaq is impressive, but it was a different game. A lot of our basketballers today have a lot of muscle but it’s not clear that it really helps. The great shooters are still puny. . .Hamilton, AI, Reggie Miller, Gilbert Arenas. Some of those old clips of centers seem to indicate they had a lot of game away from the basket, like Nowitzki or Garnett.
I think football guys would really dominate, but I don’t know aobut the others.
I think you’re overrating Ray Lewis by orders of magnitude there.
I think it would be more interesting to send some barely-good-enough-for-the-majors, Mendoza-line-hitting schnook or the last guy on the end of the bench for the Atlanta Hawks back in time. It would be fun to see them actually considered a star player for once.
You can’t run the ball on him today.
You couldn’t have 50 years ago either.
I understand that someone from Illinois is probably sore that Chicago can no longer claim to have fielded the best middle linebacker ever. . .at least not without simultaneously claiming a mental disorder.
Actually, they removed the compulsory figures from figure skating because a lot of the modern star skaters just couldn’t do them. They’d be amazing on the jumps and spins, but couldn’t stay on the line for a figure 8.
I’m not sure if Kwan was one of the ones who had that trouble, but if she was, she wouldn’t have a shot.
But track and field. The long jump has gone from 25’8.25" in 1956 to 28’2.19" in 2004 (Olympic Gold medal winners) - high jump 6’11.25" to 7’8.91" in the same years. The marathon has gotten 15 minutes faster. The hundred meter dash from 10.5 to 9.85.
Completely blow the competition away.
I’m not from Illinois and I’m not a Bears fan. I just think he’s overrated.
You couldn’t run the ball on him from around 2000-2003. Now he’s open for business. Fifty years ago he would have been a monster. Up until '57, when Jim Brown came.
I’d send Barry back. Quickness, power (5’8" 203) 4.37 40 yard dash
cite:http://www.profootballhof.com/hof/release.jsp?release_id=1206
Moss would have some issues with the way recievers were defended back then. Pass interference as we know it didn’t exist.
Oscar Robertson’s stats in the sixties gives a pretty good idea of what Lebron could do if we sent him back in time. (30/10/10) But Shaq would destroy the NBA of the fifties. The game was in the lane, and you don’t get much better than him two feet from the basket.
If I wanted to shake up the world of baseball I’d send a good pitching coach back. Every team had a lot of talent, so you’d do best to maximize what you have while minimizing injuries.
I don’t know that much about football history, but that’s roughly what I had in mind.
Shaq vs. Wilt Chamberlain vs. Bill Russell - especially if we’re giving all three guys equal training and conditioning in the '50s - would be very interesting. I’m not convinced Shaq could dominate those guys, and I don’t think he could ever average 50 points per game for a season like Wilt did.
I wonder about the diference in officiating. The 1950s NBA refs would call travelling on what today’s players call a jump stop, and MJ and James signature drives to the lane would be looked askance at by the refs too. Elgin Baylor was probably every bit as good as Jordan and James, and while Elgin was a great player and a perennial all-star it is not like he was a man among boys.
I digress from the OP, but I don’t think a modern NFL lineman would fare very well in the 1950s. Lineman are much bigger now, that’s true, but lineman back then were tougher, meaner and dirtier. They played hurt and they played every play, because they were in fear for their jobs. Leg whipping wasn’t illegal, it was a practiced technique. A modern lineman coming into that atmosphere would be in for quite a culture shock.
“Coach, I’ve got turf toe. Put me on the IR.”
“What’s the hell is an IR? Get back out there or you’re cut! We can find a guy to replace you on the next beer truck that goes by the practice field.”
Players are a lot more specialized today too. Squads are bigger and teams have insider linebackers who only play on 3rd and long. Back then they played every down unless a bone was sticking through the skin, and even then Lombardi would only take them out long enough to stop the bleeding. 
A modern lineman, stripped of access to his training equipment and nutritional supplements, wouldn’t necessarily dominate a player like Art Donovan or Big Daddy Liscomb (sp).