How to Read:
The character is obviously which character the matchup is about. The matchup is a bar with a 20-point scale going from -10 to +10 (where +/-10 should never, ever happen in any even somewhat balanced game). Points are easily translatable (0 is 50:50, +2 is 60:40, etc.), and no matchup below +/- 4 is "unwinnable," though 3's will be rather difficult to win. The Sheik matchup is what it looks like to completely avoid using Transform, while the Recommended Matchup is what the best choice would likely be to get an optimal matchup. If you are unable to determine the specifics of handling the matchup with the given approach you may refer to popsofctown's Zelda/Sheik Selection Guide.
A matchup may only be revised if a member from both involved character boards agrees to the new number. Feel free to debate about that number in this thread, or in some thread provided by either this, Zelda, or the mentioned character's character board.
I realize that some matchups might be equally good/bad for Sheik, Zelda, and both. Nevertheless, this is the Sheik boards, so in those situations I'm of course going to recommend using either Sheik or both over Zelda.
DETAILED NUMBER RATINGS:
0 - Even
1 - Slight advantage
2 - Advantage/Soft counter
3 - Clear advantage
4 - Hard counter
5 - Very poor matchup
6 - Extremely poor matchup
7 - Unwinnable in most circumstances
8 - Difficult to take a single stock off
9 - Unwinnable in practically all circumstances
10 - Reserved for vs Mewtwo matchup
Sheik Matchup Summary
Counter Matchups
Fox
Ganondorf
Favorable Matchups
Captain Falcon
Falco
Ike
Link
Lucas
Ness
Pokémon Trainer
Samus
Sonic
Wario
Wolf
Even Matchups
Bowser
King Dedede
Mario
Olimar
Pit
Sheik
Toon Link
Yoshi
Unfavorable Matchups
Diddy Kong
Donkey Kong
Mr. Game & Watch
Jigglypuff
Kirby
Lucario
Luigi
Marth
Meta Knight
Peach
Pikachu
R.O.B.
Snake
Zelda
Countered Matchups
Ice Climbers