My least favorite stage is New Pork City, mainly because whenever you fight Ness and/or Lucas in the 1P modes, it's always there. The cpus' AI and the insane size of the stage prolongs the match to a high degree, which hinders your time score and your chances of fighting Crazy Hand in Classic mode. Fighting team Ness or team Lucas there is just the worst <_>
When it comes to my least favorite tourney legal stage for competitive brawling, my least favorite is Pictochat. I simply do not like the transformations there and I do not fare so well on them.
Also, to the two who disliked Norfair because it's difficult to get inside the capsule when the large lava wave comes, you don't need to get inside it to avoid the lava. You can simply shield the lava, or if you don't want to damage your shield and have precise enough timing, you can sidestep it, rolling dodge, air dodge, or use a move that grants your character invincibility frames.