Old Comiskey Park, Wrigley Field, old old Yankee Stadium, new old Yankee Stadium, Shea Stadium. And if “been to, but didn’t go in” counts, Veterans, Municipal, and Fenway as well. First game attended, Hat Day at Yankee 1970, lost to Baltimore. Last game, Hat Day at Yankee 2001, probably played Oakland.
I’ve been to four, but two aren’t used for baseball anymore. (One doesn’t even exist anymore, I think.)
Candlestick Park – Giants
Oakland Colliseum – A’s
Jarry Park – Expos
Olympic Stadium – Expos
In 1993 I saw ballgames in every major league park. At the time there were 28. A lot of new parks have been built since then, but I’ve been to only two: AT&T in San Francisco and The Ballpark in Arlington. My two favorite parks on the trip were the newest (at the time) and one of the oldest: Camden Yards and Wrigley Field.
I’ve been to 20 so far. 13 are currently active.
Fenway Park (Boston)
Wrigley Field (Chicago)
US Cellular Field (Chicago)
Miller Park (Milwaukee)
Busch Stadium (St. Louis)
The Rogers Centre (Toronto)
Kaufmann Stadium (Kansas City)
Tuner Field (Atlanta)
Camden Yards (Baltimore)
The Great American Ball Park (Cincinnatti)
Jacobs Field (Cleveland)
PNC Park (Pittsburgh)
New Veterans Stadium (Philadelphia)
Seven are since defunct:
Shea Stadium (New York)
Yankee Stadium (New York)
Olympic Stadium (Montreal)
Candlestick Park (San Francisco)
Veterans Stadium (Philadelphia)
Memorial Stadium (Baltimore)
RFK Memorial Stadium (Washington)
This year I’m hoping to do the two new New York parks, and also Tropicana Field in Tampa Bay. My favorite so far has been Jacobs Field, and I don’t really even know if I can articulate why. PNC Park had the best view, and Wrigley/Fenway the best atmosphere, but somehow I really liked baseball in Cleveland. The only park that I actively disliked was RFK, which was like a combination tomb and swamp disguished as a sporting complex.
Let me know when you’re down; at the least I can offer advice on what to see and do around here.
Current:
New Busch Stadium
Kauffman Stadium
Dodgers Stadium
Fenway Park
Wrigley Field
Defunct:
Old Busch Stadium,
Angels Stadium (whatever it was called)
Metrodome
Memorial Stadium, Baltimore (no longer there)
RFK Stadium, Washington, DC. (no longer used for baseball)
Yankee Stadium, NYC. (no longer there)
Shea Stadium, NYC. (now a parking lot)
Jacob’s Field, Cleveland
Tropicana Field, Tampa Bay, FL.
National’s Park, Washington, DC.
Camden Yards, Baltimore, MD.
Petco Park, San Diego, CA.
Citizens Bank Park, Philly.
I’m not a baseball fan but I voted anyway.
2.5 - I’ve seen two MLB regular season games: one in Toronto and one in Houston (at the Astrodome). And one spring training game in Florida.
I’ve seen games in 30 parks, of which I believe 12 are defunct. This year I’m going to Washington and possibly new Yankee Stadium so I’ll move into the top bracket in this poll.
Royals Stadium
Kaufmann Stadium (hey - since the remodel last year, it’s a brand new park)
Wrigley
Comiskey (new)
Great American Ballpark
Nationals Park
Been to, but not for a game:
County Stadium (filming of Major League)
Coors Field (tour)
Tiger Stadium
Comerica Park
Bank One Ballpark Phoenix
Busch Stadium II (One before current)
Anaheim Stadium
That’s five though I voted 2-4 in error.
I’ve only been to 4, and 3 of them no longer exist! 
Shea Stadium
(old) Yankee Stadium
Veterans Stadium
Turner Field
Just two. Olympic Stadium and new Yankee Stadium. There’s been about three times in my life where I was very close to going to Fenway, but it never panned out. As a Red Seox fan, this makes me angry.
Astrodome & Minute Maid Park
Arlington Stadium & The Ballpark at Arlington (season tics)
Wrigley Field
Fulton County Stadium
Angels Stadium
Candlestick Park (last weekend ever used, Giants / Dodgers)
Great American Ball Park (just visited while in town to see it, no game played)
- Most gone.
Existing:
Wrigley
Cellular Field in Chicago
Petco in San Diego
Skydome or whatever they’re calling it now in Toronto
Recently gone:
Metrodome
Yankee
Shea
Gone for a little while:
Veterans in Philadelphia
Busch II (the one built in 66 or whenever)
Tiger
Cincinnati (I think it was called Cinergy Field then)
Olympic Stadium in MTL
County Stadium, Milwaukee
Gone for longer:
Exhibition in TOR
Comiskey
I also saw a minor league game at Fenway, but as per the OP that doesn’t seem to count.
I’ve only been to 2 MLB games in the last 10 years, though.
Growing up in New York City, naturally, I went to Shea Stadium and the old Yankee Stadium many times.
Since moving to Texas, I’ve been to the old Rangers stadium and the current Ballpark at Arlington. I also went to the Astrodome a few times.
Two major league, one AAA minor.
The minor league one was the old Buffalo Stadium on the Gulf Freeway here in Houston.
The first major league one was the Astrodome, a truly incredible place. The second was the piece of shit they built in downtown Houston and abandoned the Dome for.
Shamefully few…
Yankee Stadium
Shea Stadium
The Skydome
Camden Yards
Kauffman Stadium
Busch Stadium
Pretty much.
We lived in the far western suburbs of Chicago (Lisle, specifically) until I was 10. We rarely went in to the city for anything; the one White Sox game which my father and I went to was on comp tickets he’d gotten from one of our neighbors. (The only other pro sports event I went to as a kid here was a Chicago Cougars WHA hockey game.)
On my 10th birthday, we moved up to Green Bay (my father’s hometown). At that point, we started doing a once-a-year trip down to Milwaukee for a Brewer game. OTOH, I was also going to 2-3 Packer games a year…
I moved back to Chicago in '89, when I got a job here after graduate school. I’ve been to a few Cubs games over the years, almost always because someone else (usually a supplier) was buying the tickets. I’m not much of a Cubs fan. 
Fenway
Old Yankee
Shea
Cellular One/New Comiskey
Wrigley
Dodger Stadium
Angel Stadium
and what was the name of the old Cincinatti ballpark? I was there as a kid.