I think some of the best rooms are the ones in the MvM maps, Valve seemed to put a ton of effort into making the MvM spawn rooms interesting. I'm guessing because less players spawn in then a PVP map they are able to put in more props without affecting performance too much.
As for PvP maps I think Thunder Mountain has some cool spawn rooms, with the swimming pool and the mercs planning out their attack. I also like how open the first blu spawn of upward is, though the invisible wall/sky box is way too close and I don't like how you can't fire off rockets/grenades at the stuff in the distance.