kid icarus is a really cool and unique game. i found the online wasn't for me, but it has a huge single player campaign. it's great if you like pure action.
mariokart 7 might be the best MK game but the online is tailor made to hold good players back. when you're winning the item spamming will frustrate the hell out of you, but when you're losing it can be fun. it's a great game for playing with friends, not so much randoms IMO.
3DLand is an extremely good game, but like you said, you want online play and it doesn't have any. you should definitely be sure to pick this up later at some point if you pass it up now.
revelations is another must have, even for the graphics alone. raid mode is fun but the level up system is a little confusing... that's a polite way of saying that it's a colossal clusterfuck of bad decisions. bit of a rant here, but anyone who's played online will know what i mean...
you can't just keep the guns you have and level them up. instead, your character has to level up first, then you'll be able to buy a higher level version of the same gun you already have. you have to do this over again to keep your guns current. this means that at any given time you can have dozens of copies of the same gun, all at different levels, so of course you'll end up deleting the lower level versions to keep the strong ones.
if you play online with people who are only at the same level as you then it's no problem, but players at lower levels will have weapons that may only deal 1/20th the damage of yours. this means that you can kill a zombie with one shot while your partner has to use a whole clip, so obviously you'll ruin the match by being an overpowered asshole the whole time. you either have to lower your own level manually (and lose the ability to use your high level guns) or keep a shitload of lower level guns on hand in order to play, which i doubt anyone ever does.
it goes the other way too; if a low level person joins a match you're hosting on one of the more difficult matches, their guns won't even put a scratch on the enemies and they won't survive very long.
like i said, it's confusing. balancing the difficulty between all the different players, weapons, and levels is needlessly complicated. i've spent far more time playing the mercenaries online and to be honest i think it has better online play, but there's no single player there. raid mode will definitely net you a lot of time outside the single player game, at least.