For me, PNC is the nicest park I've been to, and it's not particularly close. I haven't been to as many parks as others, and I'm biased, but PNC is awesome.
I personally can't stand Miller Park. I think the place is a dump. I've never been to GABP, so I can't speak to that. But, I used to live in Denver, so I've seen quite a few games in Ccors. I really like the stadium, but the Rockies fans are kind of baseball clueless I've found.