Zangeki no Reginleiv has a 4 player online co-op, played it for month, lots of fun, and there are a lot of levels, but its in japanese, but not hard to figure out, just watch out for some guides.
Valhalla Knights: Eldar Saga has 2 player co-op, also fun, but i got a bit bored of it after a few weeks.
Grand Slam Tennis was also fantastic, still play it from time to time, great M+ experience.
Also if you like soccer play pro evolution for the wii, for me it was the best soccer experience ever.
also NHL 2k10 had 10 player onlne mode, were everybody controlled one charakter, really fun to try that out for once, and also had m+ suport
Endless ocen 2 is also a great game with 2 player online, one of the best chill-out games.
Also Tatsunoko Vs Capcom, loved that game and if you into fighters, there is a good community out for it
Edit:Forgot about wiiware.
Texas Hold'em Poker - Goood chilling game when you bored, perfectly before you go to bed
TV Show King 2 - If you into quiz games it has 4 player online mode, quit fun, and atleast your brain is used a bit
Uno - same as texas hold'em great before bed chilling game