Nice your thread has kind of made me want to play MK online again soon. Smash was a little disappointing to me consider there is a little lag and I can't have lag while playing Smash. Battalion Wars 2 was a little fun online but that was at least a year or two ago and I am sure there are probably only a select few still playing that online right now. Monster Hunter 3 will probably be a game I play the most online on the Wii. If you don't have Monster Hunter 3 then I would suggest you to get it. The offline and online modes are a blast in that game.
I have played World at War and Black Ops a lot online on the Wii also. Black Ops in my opinion is a better online experience than Goldeneye (easier to add allies/random good people, supports mic, has a little more options, etc..) However, I was having problems with host freezing and frame rate issues. They have fixed most of the freezing and the host frame-rate issues aren't game breaking but it does make me want to not be host sometimes. I prefer the way the Wiimote+nunchuck handle more on BO than Goldeneye.
I also played a lot of Animal Crossing online when it first came out. After about a year of playing and pretty much unlocking almost all the items, buying all the upgrades for the 4 houses, collecting almost all the bugs and fish, etc... I kind of stopped playing it which happens to me for all the Animal Crossing games.