I fiend Sheik to be Link's hardest match-up. She has a bigger grab range then most characters of her run speed, so she is abnormally good at getting in a grabbing Link and can do lots of mean things once she grabs him. She's also at this weird weight and fall speed that makes extended combos not work so well. Her needles are particularly annoying because they don't clank, she can throw them past your projectiles and keep you stunned while she quickly closes in the gap. She gets really juicy combos on Link that always ends in a f-air that you can't really escape, sending you straight side ways. With a mix of grounded and aerial needles, she can proceed to harass your bomb-jump recovery, making coming back a chore. Also, Link being one of the slowest characters has no reasonable option to stop Sheik from doing the ledge stall infinite. I've had Sheik players harass me with that to the point that either I put myself in a terrible position (and often just lose the stock as a result) or accept that I'm being timed out. This causes losing the lead to not be an option vs a lame Sheik.
Falco is pretty hard too, but there are a lot of tricks that make the match-up pretty manageable. Don't try to directly fight against the lasers, but don't sit there and take them. The goal is to never be horizontally aligned with Falco unless you have some sort of an advantage. If you have a bomb in hand, you can bomb toss oos fast enough before Falco can laser again (depending on how far away he is). Also don't forget that your bombs, arrows, and boomerangs go at angles, so being slightly above him, like on a platform when he's on the other side of the stage is preferable.
Falco's advantage in the match-up is the dominance of the neutral game he initially gets with the laser and the incredible shield pressure, but if you play it real safe and stay mobile, you're going to force him to go for some soft-reads which will lead to some mistakes, and spacies get punished hard for mistakes.
Don't forget the magical percent. At around 80%, d-throw auto-combos into up-b against space animals. Link's n-air off stage kills Falco's up-b recovery for free. You can also do this to his forward-b, but it's a little harder. Make sure to practice that.
A really patient Falco can take advantage of a huge stage layout and try and camp you back too. Vs Falco, I try to avoid stages with high middle platforms like Dream Land or Battle Field. However, you do need at least one platform to get a breather from lasers, so I would be reluctant to go to Final Destination too. I prefer a stage such as Pokemon Stadium, Green Hill Zone, or Wario Ware to take Falcos.
Hmmm, I guess I'll update the match-up thread with my Falco advice. Wish I had meaningful advice for Sheik, but all I got now are issues Link has dealing with her. Maybe another day.