I think New Leaf is the best simply because of the graphics and how you get to be mayor.
Although I miss the old villagers that were only available in the old AC games.
I would say New Leaf is definitely my favourite. I would then rank Wild World and City Folk on the same level purely because I haven't played too much of either / can't remember much of it. I think I liked City Folk having different characters in different houses because that was a large problem I had with my brothers on Wild World.
I haven't ever played the Gamecube game but I hear it is really good. I'm not sure if I'd like to go back and play the others because New Leaf just feels so much better than all the rest. This is the first game where I really cared about my town and dream villagers and wanting to get hybrids and decorate my town with PWPs and have a great house. The island and mayor aspects of it are just so much better than what earlier versions were like.
I personally enjoyed the GCN version more than New Leaf, but there's no denying that New Leaf is the best in the series - I just don't play games like I used to. It's still far from perfect, but it does very much exceed the GCN version in nearly every way.
Wild World was a mix. It did some really good things (hairstyles, accessories, Wi-Fi, etc.) and then some really bad things (removing holidays, messing with villager interaction).
City Folk was just straight-up a very poorly done iteration. Everything they did wrong in WW (with the exception of holidays) was made even worse in CF, probably due to laziness. The game had some neat little features added, but they suffered from poor execution. This game looked shiny, like it was the return to the greatness of the GCN version, but that was a lie.
In terms of how much I enjoyed them?
1 - ACWW. Simply because it was my first ever AC game and it holds a lot of nostalgia. I honestly lost myself for days in that game in ways I just can't do any more.
2 - Probably a tie between ACNL and ACGC. New Leaf has more to do but GC was so.. cute.
3 - ACCF. Just never got into it. Probably played it about three times.
Tied 1: ACNL and ACWW. ACWW was my first ever nintendo game, and it holds many years of fond memories. I dig it out to play sometimes.
2. ACGC I hear its really good
3. ACCF. I got it as a present but never got into it. It was just clunky having to use a TV when I can easily pick up and play a DS, and stuff like KK was hard to do because others wanted to use the TV at the time. Must have played it less then 10 times.
GC wins because it's the first one I played so everything was new and exciting and the nostalgia...
ACNL is next because the graphics are nice, there's multiplayer, the customization of the town is awesome, and QR codes, etc. allow for more customization overall.
CF is next because I enjoyed playing it online with my husband when we were in a long-distance relationship for 3 years. So it was sort of special to me.
WW is last only because ACNL is a better version of WW and WW has nothing too fantastic about it in my opinion.
New Leaf for sure, but one thing I LOVE about say the gamecube one is that the villagers are so much more unfiltered haha. Dang, I have this cranky pig in my GC game, Boris and he made me feel SO bad about not playing for a month haha. I was so annoyed at him that I just didn't talk to him after that xD