vs mk imo you should always be playing sheik
so long as you angle the shield up they can't use tornado as an approach or defense tactic against you (one grab = 40%), sheik has needles to shut down tornado at most ranges
sheik doesn't get gimped AS hard as zelda imo, you just need to be more creative with your recovery
ftilt -> up smash *****
sheik's jab cancelling -> grab on the shield is safer than any of zelda's options if you do mixups
sheik's needling is one of the FEW moves that's guaranteed damage on MK off the ledge...if MK approaches from like below the edge (he really shouldn't be doing this though) or at it from the side you can just throw needles. actually if you feel like having some fun you can just toss needles horizontally if you predict MK will come into your range off the edge, one or two is enough to throw him off
sheik's throws essentially all set her up to charge needles and run away or if you condition them into an airdodge a nair is nice, MK at any given point has few options vs. a needle camping sheik, if done correctly the MK is forced to approach at all times (just make sure you're throwing at near max distance or you walk -> needle, shielding the needles will stun them just enough where if they come at you from that huge distance you can easily just kinda run around and jump...just pick stages with low number of platforms like FD, SV, YIBrawl..)
sheik *****, guaranteed kills on metaknight at percents that most characters dream of killing any high tier at is just awesome
i'm pretty sure most of this is right based on my observations of needles and general applications of anti-MK strategy to sheik tools...NL confirm this or show vids? <3
edit: grab release -> grab release is possible i think, you gotta be kinda hasty about it though i think