I've been to 24 current (AT&T Park, Camden Yards, CitiField, Coors Field, Fenway Park, Globe Life Park (The Ballpark in Arlington), Kauffman Stadium, Miller Park, Nationals Park, Oakland Coliseum, PNC Park, Progressive Field (The Jake), Rogers Center, Target Field, Tropicana Field, Wrigley Field, and Yankee Stadium III) and past (Busch Stadium II, Metrodome, Olympic Stadium, Sun Life Stadium, RFK Stadium, Shea Stadium, and Yankee Stadium II) baseball stadiums, plus Field of Dreams, City of Palms Park and I'm visiting at least two Cape League parks this summer.
I have a natural bias towards Fenway. It's where the home field of my favorite team, I saw my first game there and it is by far the park I've been to the most. Discounting that the parks I liked the most (besides Fenway) are probably Camden Yards, AT&T Park and Miller Park, with Wrigley a close 4th. Camden is Fenway South. The question on our particular Southwest flight was not, "Which game are you going to?" but, "How many games are you going to?" AT&T is picturesque and Miller is very comfortable to watch a game. I think maybe the seats are a tad wider to accommodate for the typical Wisconsin diet. We saw a game from the Right Field roof top at Wrigley and the next day from the Bleachers. Cubs games are an excuse to drink in the early afternoon. The game (and whether the Cubs win or lose) seems secondary.
By far the worst are the Oakland Coliseum and Stad Olympique (as the Quebecois refer to it). The Coliseum is not in the greatest of locales and, unlike Camden Yards, the stadium itself does nothing to make up for it. Stad Olymipique was just a complete Sh1thole. I went with my buddy on July 24, 2004 (Red Sox fans and A-Rod know what happened that day) and it was quite apparent why Major League Baseball was leaving Montreal less than 10 weeks later. I literally had my own bathroom and went undisturbed for a half-hour (must have been that rich French food). Busch Stadium was my first AstroTurf game and first in a multi-purpose cookie cutter stadium. The Metrodome was my first indoor game. I didn't like either. They didn't have any of the aromas I'd come to expect at a baseball game. RFK had the feeling of a football stadium and is also not in the greatest of neighborhoods. The only reason it is not among the worst I've been to is because it is the former home of my favorite football team and I was happy to see an event there.
Regarding the New York stadiums, CitiField blows Shea out of the water. It really is no contest (Shea was a multi-purpose cookie cutter stadium. Are you seeing a trend yet?), but I felt that as a baseball stadium (as opposed to a revenue generating attraction) Yankee Stadium II didn't really need to be replaced. Unlike II, I have no feeling walking into Yankee Stadium III.