What is the best MLB ballpark? What is your favorite MLB ballpark? What is the worst MLB ballpark? What is the most over rated MLB ballpark? What is the best looking MLB ballpark?

For me.
Best - PNC - right in the heart of the Pittsburgh. Beautiful place with a pretty view of the city. Coors and SF close second.
Favorite - Yankee Stadium
Worst - Tropicana
Over Rated - Fenway
Best looking - PNC. Citi Field, Coors and SF close second.

Best: Fenway. Only Wrigley is close.
Best "new" park. Pac Bell
Worst: Tampa

Others I like: PNC, Camden, Safeco

for me wrigley beats fenway only because the seats are arranged to look at the action
fenway close second
worst:  U.S cellular so hot no wind i thought i was in hades
best new Busch stadium 2
others: atlanta was nice,kauffman always friendly

BEST-- PETCO

WORST--TAMPA

I never understood the appeal of Fenway. Perhaps the only thing worse that that green wall is that silly hill in the Astros' centerfield, which I've heard is being removed after this season. Just because it's old and odd shaped doesn't make it a good ballpark. And I never felt it had the same mystique and magic to it as the old Yankee Stadium did. I don't think the new Yankee Stadium has that mystique to it. It's almost too massive. But hey...it's subjective...no right or wrong answers. And certainly a lot of people do like Fenway.

Best : Comerica (because of restaurants around the stadium)
Worst : US Cellular
Best looking : Minute Maid. Awesome Hill (plz don't go, some of the best moments happened on that hill ex, Andruw Jones on back to back plays had to run on it to make the catch, tripped on first attempt and missed, so funny) Train that USED to move on Astros homeruns.
Overrated : Wrigley
NO ONE TAKES TAL'S HILL WITHOUT A FIGHT. BLOOD WILL BE SHED FOR IT!!!!

Haven't been to 'em all, but been to a lot.  I'll start out with the worst.

Worst:  Oakland,   ever since the park was remodeled the view to he Bay it's almost depressing to be there. Plus the walkways are chipping away.  Those things and the fact the concourse area is cramped and cloistered.
Best:   Pac Bell,  a destination location. Great seats, better view.
Favorite:   Fenway,  cool place that is steeped in history.  
Overrated:  Tough call, pass
Best Looking:   Pac Bell, inside, outside, and view shed all great. 

Grew up in L.A., so I am biased;

Best - Dodger Stadium...Clean, and the whole place just screams Southern California

Worst - Oakland (post remodel)...Just a mess on so many levels

Overrated - Pac Bell...Don't like the access, (lack of) parking, or the 'feel' of the park...The 'anti' Dodger Stadium

Most unfairly maligned - Candlestick...Night games there could be a real nightmare, but during the day, just a great place to spend a sunny afternoon...

Favorite: Safco is beautiful! Can't hit worth a darn at that location but well maintained park!
Loved those days at Candlestick with terrific view of the bay before they blocked that out...talk about cold tho...

I agree with cwillis, nothing compared to the old Yankees Stadium. Sob. Okay, I am sort of over it. Let's move on. 

Best is probably Fenway

Worst is anything with a corporate logo for a name

and yes Candlestick was unfairly maligned, I thought it was a beautiful park.

under-rated: Coors - I have been to the ballpark many times (my sister lives in Denver) and it is really charming, the whole offense thing aside. 

Nicest looking: Camden Yards

Honorable mention: Tiger Stadium, though it started to get uninhabitable a while back. But in its heyday it was something and I can still see Reggie's All-Star game blast hit the top of the roof in 1971.

Let's remember: Ebbetts Field

Why is Fenway (and Wrigley) great? Every ballpark should be distinguished by oddities, local home field rules, reminders of place and that baseball is not mass production, not standardization, that it comes from what Greil Marcus in "Invisible Republic: Bob Dylan and the Basement Tapes" calls "The Old, Weird America". 

When I walk into Wrigley or Fenway I see hack Wilson Rogers hornsby Ferguson Jenkins or babe Ruth tris speaker fred lynn that's as much the beauty for me as the park itself I love the new stadium but long for the old

Wrigley Field and Wrigleyville combined to make an experience that's difficult to recreate.  

as for the places I have visited,

lifelong boston lad, 1st fenway visit 1979, it has been tweaked over the years to fit it into the national spotlight, those of you who have been there,will co-remember some bad times, but still my fave,,,

A:Camden,Wrig(see above), and San Fran,,,

B: Pitt, St Louis, Atlanta

C, Minny, both ny's , (new yankee....bleh)

D: Oakland, a concrete mess, skydome, a mall, yay canadia

I've only been to twenty-odd parks, most of which are not in use any more.  I agree with a lot of the comments above, though.  Dodger Stadium is a classic design, and very well kept.  I'll take Wrigley over Fenway any day: much prettier, better sightlines, better fans (maybe I caught the Sox on a bad night, but the fans were horrible) - the feeling of history cannot be replicated.  I'll always like Camden although I haven't been there, because it was the first of the 'throwback' stadiums with charm and character, not the soulless parks of the 60s through 80s.

Worst park, hands down, for so many reasons: Commiskey.  Followed not too closely by the Oakland Mausoleum.

Yankee Stadium is in the Best class by itself, but I'm prejudiced.

Of the ones that aren't in use any more, Tiger Stadium.  Old, dirty, but the first of the real old stadiums I ever went to.  Definitely got the sense of history rush.