Everyone is going to have a favorite- and they're all different. My advice is to get together with some friends and try the different taster packs (or just get some people to invite you to their parties) and see which one you like best.
Cubi Racing: Lilliputt (cute windy path with spectator platforms above), Winter Wingding (long flat bottom level, roof and trees to spectate).
Cozy socializing: Mazza'la
Of course these are just my opinions. I've seen great races at Double Rainbow and plenty of socializing underwater in Nylon Phool. Your mileage my vary.