FubarThis ctf talk brings back memories from 2010. Tf2maps.net had a "make ctf for comp" contest and it was kind of a crapshow. https://wiki.teamfortress.com/wiki/Competitive_CTF_Contest That said, what about ctf_landfall?
Landfall is one of the least obnoxious CTF maps I've ever played but it still has some really bad chokes, and the only non-interior passages that wrap around the map are very narrow. The spawns are also placed just a little too awkwardly - plus it has two spawn rooms, which frankly I don't think a 6s map should have. I really like the mid and the intel area could definitely be rearranged a little and it would be great, but the biggest issue for me is the pathways between them. It's like what would happen if you took Badlands and removed the entire train path to second.
Landfall and Doublecross could both be somewhat decently modified with pro versions for 6s. Sawmill also wouldn't be that obnoxious if the ctf version didn't block off passages so that you have to walk through the stupid center, if the intel wasn't deep below ground protected by the enemy spawn, and if the mid didn't have the sawmills as an actual hazard (they could be stationary and that would be fine).
Well is too big, and while I like its intel room/final cap room (although the pathways are too high up for non-explosive classes), the spawns are too far forward and it's way too chokey. 2fort is hopeless. Turbine is really only suited towards HL.
Funny as it sounds though, now that I think about it, it seems to me like the CTF map most ready for 6s is actually Thunder Mountain, assuming all of the mannpower junk was removed. It's not actually that bad once it's mirrored like that, and the spawns are far enough out of the way that you could still run in and capture without having to wipe the entire team. Gorge might also work well (its mannpower variant).