Just to keep things varied I'll mention customs only:
1. labor: hands down my favorite map. Often get a lot of epic rounds on this map; it's not terribly difficult to assault the center point with a decent rush, which makes it more enjoyable and keeps the battle changing between a fight for the center and a fight for the 2/4 point.
2. stag: lots of levels to fight on, lots of flanking, it's favorable to pretty much any class without having a single class being too dominant.
3. castle3: I'm still partial to the 3rd version of the map, 4th version just sucks once you get to the last stage.
4. boulder: Haven't played it a lot yet but I'm liking it more and more as I learn all the routes, especially for the last stage. I really don't like the last stage but only because it's possible for the attacking team to circle all the way around the left end to the defending team's spawn and camp it with a height advantage.
5. corporation: Again, I liked v2 better than the current v3. stage 2 is a lot of fun as a soldier, stage 3 is just chaos which is great when you get massive pushes and defenses that just barely hang in there.
I was looking through my map folder just now and noticed a lot of maps we don't play anymore: aqua, desert fortress, impact, mach4, corporation, circlejerk, canalzone, chaos, jailbreak, roswell... and there's a few I've played on other servers that may deserve a look into: egypt (which we played once on our server) and crossunder.
To be honest I enjoy all of the stock maps because they're all different enough that they're a ton of fun with the right teams. I never blame a map for a bad round, I'd much rather blame the teams first (and not just skill balance wise, but also a lack of teamwork and aggressiveness and class choice). People complain about hydro, turbine, and 2fort but I really don't see why :\ Think your team isn't pushing out? Be a medic and get those ubers to help break out. Hydro locked in a stalemate all the time? Play scout or spy, flank around and cause chaos in the backfield. You don't have to always say 'omg team can we please push out?' Just say 'hey demoman, I'll uber you and we'll take down that sentry outside the door, pyro follow us and clean up once it's down' It helps if you're a lot more specific and it's a lot more pleasant than hearing generic whining about 'the team' in general
... wtf how did my post end up being this much offtopic