Obviously I am biased, but GT takes my top spot. That said, I have been to almost all of the stadiums in the ACC at least once. In fairness, a few of those games, (such as Syracuse, VT, Miami, and Pitt), I didn't see GT play, but rather saw a college football game with a friend who lived in the area of the team. Here's how I would rank them.
#1 GT by a landslide.
#2 VT It's close, but it's the second best experience I've ever had at stadium. The fan base is into the game start to finish, and the pregame is awesome too. Fans are a bit pretentious for a team that has never won anything of import though.
#3 Clemson Friendly fans, awesome tailgating, great atmosphere. Seating is awful though, and they really need more than one road into and out of the stadium.
#4 FSU Horrible location, hot as #$^%, ignorant fans, but great atmosphere during the game
#5 NCSU Really just a great venue. Fans were super friendly and tailgating was awesome.
#6 Wake Forest Pretty campus, great fans.
#7 UNC Better than others on this list, worse than some Pretty much mediocre
#8 SU I hate domes, but considering the late fall weather possibilities there, I'll forgive them.
#9-11 Pick an order, I've never been to UVA, BC, or UL
#12 Dook Even empty, it's better than playing in an oversized NFL stadium without the fan support to come anywhere close to filling it.
#13 Pitt Plays in an oversized NFL stadium without the fan support to come anywhere close to filling it.
#14 Miami Plays in an oversized NFL stadium without the fan support to come anywhere close to filling it and their fans are largely classless jerks that cheer injuries.
Caveat: We aren't including ND it appears, but if we did, they would be a solid #2 and everyone else would move down accordingly.