@DACIT comment: I don't know if its the best, but he does still have his from Brawl yes. Harder to pull off, but still amusing. Particularly against Diddy. DACIT -> Banana hitting -> Reverse Fsmash for the KO was a thing that happened in tournament back in Brawl.
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Sheik "invalidating" Ike: You should ask VoiD how he feels about that after he lost to SM's Ike. Its 40-60, that's not invalidating.
A super super simplified version of the how the MU plays is this: during Ike's first 80%, he's largely getting bounced around while tacking on a few random hits here and there. After 80%, Sheik's combos stop working so well against Ike, his combos are working perfectly fine, MU starts swinging towards him a bit. Within lets say, 3-4 hits, Sheik is in potential danger of dying because Ike is still at rage and not dead yet. Shortly after that point, it starts becoming "whoever can land the first solid move first takes the stock".
If Ike takes it, Sheik comes back but her combos still aren't working well against Ike so he's free to tack on extra damage until she finally kills him. If Sheik takes it, Ike has to quickly try to land a solid blow before Sheik tacks on too much damage. There is also a constant factor of "if Sheik ever has to use Vanish to recover while Ike is anywhere near the stage edge, she's most likely dead". If she gambles with recovering through bouncing fish and we read it, its a free Fsmash (has happened in tournament).
Its 40-60 because Ike's window where he can reasonably kill Sheik and she can't quite kill him yet isn't as large as we'd like it to be. She can also capitalize a bit better on taking the first stock better than Ike can. If she had a better recovery it might be a different story.
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As it stands, Ike doesn't need a secondary. Some people might opt for one, but there isn't any MU you can point to and go "Ya you need somebody else to handle them". Every MU is reasonably winnable.
I know the MU range for Ike I suggested is hard to swallow, but just remember that when I initially said Ike beats Sonic there were some doubts going around. And then I posted that set of videos, which I think is further proven by Static trying to use Roy instead of Sonic against Ryo the following time.
We're not crazy. Just not enough coverage of what we can do as some people would like.