Sheik is top of S-tier for most people because she can 0 to death combo starting from fast, safe moves, has strong recovery, and has a strong grab.
Her only downside is she racks up percentage slowly with individual hits. But it doesn't matter because you can combo for like 20% and take them off the ledge using only forward air short hops, where you can then abuse her stupid powerful edge guarding. Her back air and up air are still amazing finishing moves.
The tiers might shake up a little bit once people start learning Ryu, and Rosalina&Luma are usually considered S-tier as well, but they're both highly technical characters. In fact, most of the other top tiers are pretty technical, Sheik is probably the most braindead one out of all of them up there.