Little Mac just because of the way he punishes your everything. Ike's only advantages are the same as anyone else without a projectile. Bait, punish, and rely on gimping. You have very, very limited windows to punish anything he does out of shield, while he can punish you from the other side of the screen just for breathing wrong.
USmash is the only thing you can truly punish OoS, and that's only if he hit close. If he hit spaced and you go in, he'll punish you for trying to punish him. Ike also lacks good launching options that go horizontal. FTilt is a big no-no, Bair gives him a vertical leverage he can use to recover back, BThrow doesn't launch him far enough, jab also gives him the vertical angle advantage, and any of your other moves are just too slow and punishable to even consider.
Lastly, your spacing means nothing to him. You're not going to win an exchange due to his reach. You can't expect he's just going to FTilt into your DTilt and get a free hit. They have a much easier time looking for ways to punish you off of the wrong hit, including with a dash-pivot FSmash, which not only shifts his hitbox away, but returns with a very powerful hit, can't be punished on shield, and has little to no hitlag. Plus, anytime he gets a KO punch charged, you're in danger. DTilt, DTilt, KO Punch can one-shot you from 0% and there's no way out of it, either.
Right now, your best bet is to have Info Sharing set to On so maybe, just maybe if Sakurai is feeling generous when Mewtwo comes out, he can review the data and give the game a balance patch.