The question is unanswerable because MSC tops them both.
Brawl's online sucks because SSBB is a game that requires precise timing; even the slightest bit of lag WILL screw you over. Players have adapted to this by camping and spamming, which makes online boring lol. I mean, you can still win in it, but it's just boring if someone just takes Snake or MK and acts like a predictable ass. Also, it relies too much on defensive play and single hits (as opposed to combos) due to the lack of wavedashing, making it even more boring. It does have a few advanced tactics, but they are not really useful in most cases.
MKWii has some decent racing, but it's pure luck. Luck essentially decides who is going to win, especially in the case of Blue Shell and Pow Block. (Yes, I know POW Blocks can be cancelled, but when you get flinged off the stage in the last lap due to pow block cancel's slide effect, come back and tell me that.) And even without items, it still sucks, because there's only memorizing the racing track and bumping someone off the stage.
MSC, on the other hand, kicks ass. Items and luck are minimal due to the invention of the trickster style and the fact that luck only punishes specific moves in the game, which a player can choose not to use. Lag is minimal due to a well built online system, and when it does exist, it has only a minor effect on gameplay due to the fact that MSC doesn't take precise timing (emphasis on "precise", it does indeed have timing but lag won't change it too much). In most cases, a player with skill will destroy a player who is less skilled. In addition, because of the frantic style of MSC, you are put in a WIDE variety of situations and you are forced to adapt to them quickly. And of course, there are multiple advanced techniques that a player can use to score a goal.
I believe my point has been made. MSC kicks the asses of SSBB and MKWii, lol.