My favorite is without a doubt Wild World. It's what got me into the franchise in the first place!
The feeling i got when playing it was just unlike any other.
I felt peaceful yet excited when playing.
I didn't get that feeling from City Folk, nor from the original when i tried it.
Also i just think the way they made both Wild World and City Folk suit the DS better than a single screen.
City Folk should've been more like the original, instead of trying to be Wild World.
I ALWAYS started my day with doing my daily objectives, and i always looked forward to doing them again the next day.
I didn't do that with City Folk. Sure, it may have been cause it's not portable, therefore i can't take it with me and save time, but the point still stands.
Overall, Wild World was the one for me, and i still have it!
The 3DS version i'd like to see though, definitely looking forward to it!