Between all of the main 2D Mario series games, which is your favorite? All of which being:
Bros. (SMB1, SMB2, SMB:LL, SMB3) World (SMW, SMW2: YI) Land (SML1, SML2) NSMB (NSMB, NSMBWii)
Between World and NSMBWii, I find it hard to choose. I know the NSMB franchise is a bit revered by fans, but honestly NSMBWii is some of the most fun I've had playing a 2D Mario yet. Multiplayer is always a ton of fun to play, the secret paths and warps add great replay value (as do the Star Coins and secret world), the little mini-games that levels occasionally have make it fun to explore and take your time (Jumping on Bullet Bills for infinite 1-ups, Red Coin challenges, etc), etc. I don't remember loving the first game all too much, but NSMBWii is always a load of fun to play alone or with friends, and NSMBU looks of similar quality.
Super Mario World has a lot of nostalgia for me too, with the world map that houses dozens of secret levels, worlds, exits, etc (That I'm happy to see NSMBU trying to replicate). I haven't played it in a number of years, but to my recollection it is my favorite among the classic games.