I've only been to 24 of the current ballparks, but I have also been to five other parks no longer in service.  Here's an attempt to rank them.  The top 3 are the best, ranks 4-20 are relatively  interchangeable. 21-24 are rather dreadful.

Current:

1 - PNC Park - If there's a perfect park, this is it

2 - Oriole Park at Camden Yards - CLASSIC. Seems like it has been there for 50 years, but still new

3 - AT&T Park - Incredible setting

4 - Fenway Park - the recent renovations are terrific

5 - Citi Field - Biased Mets fan, but I love the place.  Best exterior in MLB, just need a better team.

6 - Dodger Stadium - I love the classics

7 - Wrigley Field - ditto Dodger

8 - Petco Park - Great city, nice design

9 - Citizens Bank Park - Well done ballpark.  I'd go more if the Mets/Phils rivalry were more competitive

10 - Safeco Field - Great location in a really nice city

11 - Comerica Park - Underrated, love the statues.

12 - Yankee Stadium (new) - More functional than the old Stadium, but lacks the charm. Frieze great.

13 - Angel Stadium - Best of the 60's multipurpose parks. "V" shape and SOCal weather made it last.

14 - Miller Park - The dome takes away from it a bit. Is it necessary?

15 - Great American Ball Park - Cool view of river and Kentucky

16 - Progressive Field - 3 of 4 games I've been to have been in terrible seats, nice park though.

17 - Nationals Park - Concrete/metal look seems cold.  Upper deck seems unusually high.

18 - US Cellular Field - Went there mid-renovation in 05.  Seems like a nicer park now.

19 - Turner Field - Sort of bland.  Not bad though. 

20 - Chase Field - Only went on a tour, tough to judge

21 - Rogers Centre - If it had grass it would be a decent park

22 - Tropicana Field - Dank awful place for baseball.  Tampa/St. Pete deserves better.

23 - Oakland Coliseum - At least it has grass

24 - Sun Life Stadium - just a football stadium

Out of service:

1 - Yankee Stadium - Classic park even in its post-1976 renovated form.

2 - Shea Stadium - I went to over 250 games there including my first in 1984

3 - RFK Stadium - Grass field, not much else to say...

4 - Veterans Stadium - Concrete monstrosity.  Saw 1996 ASG there though.

5 - Olympic Stadium - By far, the worst park I've ever seen a game at.  The football stadiums were better.

Getting to Minute Maid Park, and Rangers Ballpark in April.  Possibly Target Field later in the year.