I don't think I can answer this question. I've only been to a total of 4 MLB ballparks and three of them no longer exist.

(Wrigley Field, Comiskey Park, and the previous stadiums of the St. Louis Cardinals and Milwaukee Brewers.)

Photos from Ryne Sandberg's Chicago Cubs jersey number retirement ceremony at Wrigley Field

Do just MLB ballparks count?

I guess my five favorite MLB parks would be

1) Wrigley Field
2) Coors Field
3) Petco Park (despite the name)
4) Ameriquest Field in Arlington, and
5) ... hard to say the fifth. The contenders:

I thought Miller Park has a bad rap for no reason. It's a perfectly good place to watch a game, although I say that with the caveat that I wasn't in the upper deck. It looked pretty high up there. And it's certainly a unique place, with more of a modern feel than most new retro-style parks but still retaining a baseball feel.

Turner Field doesn't stand out architecturally at all, but the city of Atlanta does a really wonderful job in making it easy to get to the field if you're an out-of-towner. From our hotel downtown, we were able to take the subway to the Underground Mall, and then took a special gameday shuttle bus to the park. And before the bus left the station, an a cappella quartet got on to sing "Take Me Out to the Ballgame". All in all, a good baseball atmosphere that makes up for the average park.

Chase Field in Phoenix also has its merits, although my perception may be skewed because the game I saw there was the only time I've ever had legitimately great seats at an MLB game - second row behind the third base line, right next to the dugout. Really, the game is completely different down there. It was also my birthday that day, and I saw a really great game besides. A great day; to be honest, I don't really remember the park itself all that well, even though I remember it quite fondly.

Among minor league parks, I thought Isotopes Stadium in Albuquerque, Whataburger Field in Corpus Christi, and AutoZone Park in Memphis were the best, because they all really take advantage of their settings to be unique to the city they're located in. Isotopes Stadium feels like Albuquerque, with the great view of the mountains beyond the outfield wall. Kevin Reichard has a good review (with pictures) here.

Whataburger Field is actually built in the middle of the Port of Corpus Christi, and incorporates a few of the preexisting old buildings into the park. You can see ships going by on the canal just over the left field wall, and over the right field wall is the large bridge connecting the mainland to a few of the islands. It's a park that just wouldn't be possible anywhere else.

AutoZone park is right in the middle of downtown Memphis, one of the few minor league parks I know of that has a downtown setting. And because downtown Memphis is such a happening place, I imagine the park is almost always full, also rare for the minors. It sure was the night I was there, and it made for a great gameday atmosphere - the minors are fun, but the games rarely feel like an event like the game in Memphis did. Also, a beautiful park in its own right.

Like Diana, I've only been to four ballparks, too. Wrigley, Kaufmann, the Metrodome, and old Busch. Obviously, Wrigley is the best of those four. I also like Kauffman, having been there a few times.