Sure, he can be beaten. Then again, any character can be. The issue is: MK is hard enough to beat to make it... unpleasant to fight him. He might be a lightweight, but, seriously, if you don't get the kill when you hit him to the side, he'll be back and he could come from ANY direction.
Sonic might have options for recovering, but, quantity is not as useful as quality. MK's recoveries are ALL good, and numerous. Sonic has like... uB, sB, jump... dB isn't that good for recovery... takes too long to charge. I suppose you could spam nB, but, that'd make things worse if you were too far away from them.
DDD is slick. But, his 4 jumps are roughly the same as a normal character's 2 jumps.
Warrio gets gimped when an MK goes after him, hits him off the bike, and then laughs like a total prick while Warrio falls.
The Puff is great at recovering, but can't kill nearly as well, since they murdered the Rest attack.
ROB is a tough one. His recovery isn't really gimpable, but, whatever, that's ONE guy. Anyone else, no matter how sweet their recovery, can STILL be KOed by MK via Gimp.
And, please note, I said you can't shield the Tornado in the AIR. Once you're up there, he's got a chance to make you suffer with that thing. And, it does hurt after a while. >.<!
Also, remember: Brawl Tier List.
SS: Meta Knight
S: Snake
A: Marth, Falco, (others.)
FULL TIERS between MK and Marth. He's insane. Just freaking insane.