I've attended most of the ballparks in the country so this topic is right up my alley.
Currently living in the northern Chicagoland area. I attend about 10 games each a season for the Brewers, Cubs, and White Sox. I agree with FW that the Cell is very underrated; it is a great place for baseball. Wrigley Field has historic charm but is kind of an uncomfortable hassle to attend on a regular basis. Miller Park works just fine; the retractable roof is great. I think I enjoy the Milwaukee fans the most for some reason.
I grew up and lived in Orlando for the first 42 years of my life. Given the heat and humidity down there I could never figure out why Tropicana Field always got such a bad rap; I love that place. It could be laid out a little bit better concourse-wise but I think the seating and field areas work very well.
(This may sound awful to most sports fans but when it comes to attending sporting events I prefer domes or at the very least retractable roof venues. Dealing with weather issues, be it cold weather or heat or getting sunburnt, etc., is not my idea of a good time. Therefore I decree that all sporting events be indoors. LOL!)
The old Marlins stadium (also the Dolphins stadium) had issues but it never stopped me from going there. Can't wait to see the new Marlins ballpark; it is high on my wish list.
My favorite ballpark by far is PNC Park in Pittsburgh. I love going there and I love seeing it on TV. It's a beautiful place and has an amazing view of the city.
Another park that stands out in my mind is Camden Yards in Baltimore. I think Baltimore was one of the first to get one of these great new ballparks and they did a terrific job on it. I attended a few games during Cal's streak and those are some of my favorite live baseball memories.
Love Kansas City's park with the fountains and all that. They won't need to re-do this park it is fine the way it is.
Seattle has a great park, as does San Diego.
The park that is highest on my wish list is the one in San Fran. Saw many games at Candlestick but have only seen AT&T Park from the outside. Looks tremendous.
Perhaps the best upgrade of them all was Minnesota's move from the Metrodome to their new place. I think they should have gone the retractable roof route but what they built is pretty darn nice. Baseball in April in Minneapolis is probably no picnic, though.
If you ever have a chance to take a tour of these new parks, do so. Well worth the time.