Two of my favorite authors, Neil Gaiman and Guy Gavriel Kay. The former was a three-hour line, and by the end all of us - including Neil - were pretty much exhausted. He was still perfectly charming, though.
I know…I thought about making a comment to that effect, but I decided not to.
I guess I have eclectic taste! 
Both were awesome experiences for me.
Al Leiter, the baseball pitcher, when he pitched for the Mets, in a charity event just prior to the 2000 season. I walked by a Mets Clubhouse shop near my office during lunchtime and waited on line for about 30-40 minutes to have him sign a poster and a baseball, and have a few words with him. It was cool seeing someone I’d seen close up on a TV… up close in person.
My wife once tried to get Spike Lee’s autograph once at a basketball game. I was ready to take a picture of the event, but she was intercepted by security or by his coterie about ten feet away from the man, and turned away. So I took a picture of that instead.
Then there’s that Santa Claus guy…
Neil Gaiman. Poor man’s book-signing hand must have been numb by the end of that evening …
To get pictures of (and shake hands) with President Gerald Ford.
For Autographs - Gary Lewis, Dino Martin, Al Jardine, and Mike Love
For lecture, and book signing - Isaac Asimov
Book signing - James P. Hogan, Arthur C. Clark
Spiny Norman and I stood in this line for 2 hours to meet Michelle Obama, and got her autograph for our patience.
I’d camp out overnight to meet Barack Obama.
The only other person I’d consider standing in line to meet would be Terry Pratchett, mostly to thank him for all the joy he’s brought my husband through his books over the years.
When I was in college, I was in an organization that used to do usher duties for concerts. When Harry Chapin performed, he announced that afterwards, he’d be signing autographs and selling souvenirs to support his favorite cause (world hunger, I think?) I went down to where the table was set up, and I was standing off to the side, just to watch what was going on. Turns out, I was right by the door where he came in - we make eye contact for a long couple of seconds, then he turned and walked over to the table. I probably waited all of 10 minutes, so it wasn’t a big deal, but it’s the closest I came to being a groupie.
That was my only brush with waiting for a celebrity.
I stood in line for 4 hours to get tickets to see Bill Cosby. It was worth every minute I waited in line. He was awesome.
Garrison Keillor and Weird Al (sorta).
I met Garrison Keillor after a whatever you call what it is he does…an appearance? Not a concert, really. I was near the front of the line, so I only had to wait 15 or 20 minutes, but he took probably 3 or 4 minutes with each person and signed a book if they’d brought one (I had).
I was wearing a shirt similar to this one, only it just had the logo, not “Orange Candy” underneath. He asked me what it meant and I explained it. We also talked a bit about my being involved in journalism. Pleasant guy.
I hung around near the backstage door of a Weird Al concert before it started, hoping I’d meet him coming in or going out, but I didn’t. I had tickets to the concert and I’d gotten there early, so I was somewhat bored and figured maybe I’d just run into him.
Clive Cussler, at a book signing in Denver, for my dad’s Xmas present.
Sparky Anderson, at the ballpark. Got him to sign his book for me. Cool guy.
I think all of my other “famous people” encounters didn’t involve waiting in any lines.
Terry Pratchett - several times for book signings and such
Kevin Smith - always try to catch his gig at Comic-Con every year
Joss Whedon - The *Firefly/Serenity * panel at Comic-Con was well worth the wait in line
Any number of authors for varying lengths of line for book signings. Harrison, Turtledove, Stirling, Bradbury, Niven, Barnes, Ellison, Robinson, etc.
All sorts of autograph lines at Comic-Con.
I assume the OP is discounting all the lineups for concert tickets in my youth.
Annie Lennox!
I went with a friend who managed to finagle some post-concert backstage passes.
Annie is a teeny, tiny person. One wouldn’t know that from her commanding stage presence and voice.
Ray Bradbury is the only one I can think of off the top of my head.
I would wait to see several authors speak, as well as Kevin Smith, if given the chance.
People I’ve actually met:
Dozens of comic book creators, including Stan Lee, Howard Chaykin, Matt Wagner, Art Adams, Brian Michael Bendis, Joe Quesada, Mark Waid, George Perez, Tony Harris, Adam Hughes, J. Scott Campbell, Greg Rucka, Phil Noto, Todd McFarlane, Rob Liefeld, and a lot more.
Authors Michael Cunningham and Dave Barry.
Musicians Jonathan Richman, Lavay Smith, Pat Metheny, and the late Maurice Gibb.
Model and adult film actress Aria Giovanni.
People I’ve seen and heard (lectures but not concerts):
The 14th Dalai Lama, Will Eisner, Michael Moore, Andy Richter, Max Weinberg, Henry Rollins, Bryan Singer, Kevin Smith.
But did you stand in line for hours in order to meet/see/hear any of them? That’s the point of my question.
Um, yes, or I wouldn’t have bothered posting in your precious thread.
Mark Lenard came to the mall here back when I was maybe in 5th grade or so.
He gave a little talk, and then do autographs, spoke, then autographs, spoke, then a final round of autographs.
I stood in line each time for autographs, and they’d end it each time before I got there. The final time they cut the line off directly in front of me. I was kind of bummed, but my mom was a little assertive and I got to get my Star Trek card signed by Mr. Lenard.
Ornette Coleman
John Mayall
Stevie Ray Vaughan (waited in vain for fucking ages outside his hotel)