Hydro or Dustbowl. The balance on Hydro is all over the map, some sections are awesome, some are (in my opinion) the worst (barring 2fort, which is just lame) out of everything on TF2. Dustbowl is a little more consistent. Either way the TF2 maps are worth studying, almost all do a good job of having multiple useful paths to important points in the game, and reward those who put in the time to learn the map well without allowing them complete domination because of it.