ESPN.com's Battle of the Ballparks initially saddled Miller Park with a lowly 24 seed (out of 30, obviously), but, presumably thanks to a vigorous get-out-the-vote effort on the part of Brewer partisans, Miller was able to vanquish nine-seed Angel Stadium, eight-seed Dodger Stadium, one-seed PNC Park, four-seed Camden Yards, and, finally, three-seed AT&T Park en route to belt and title. Those plucky Wisconsinites!
Miller Park is certainly a nice place to take in a game, what with the wide concourses, retractable roof, Bernie Brewer slide, and most excellent tailgating scene. (And this is to say nothing of the credit that is due for inspiring the greatest barbecue grill in the annals of human endeavor.) However, the day-game shadows on the field are a problem for both player and fan, and otherwise I don't see how it's one of the 10 best, let alone greatest of all. Of course, such polls are close to the opposite of scientific.
I'd split the rankings into two categories: pre-Camden parks (of which there are vanishingly few) and post-Camden parks. Among the former, I'll take Wrigley. Among the latter, I'll go with PNC in Pittsburgh.