Favorite OST would be Skyward Sword. Sure, other Zelda games have specific themes that are more memorable, but Skyward Sword's OST wins out overall.

As for the other questions,

Favorite Ganon: Ganondorf from OoT/WW/TP. Hey, they're all the same character after all . If I had to pick one appearance specifically though, Wind Waker's.
Favorite Zelda: Tetra (surely she counts), for being a great character in general on top of being a refreshing change from the typical Zelda.
Favorite side character: Wind Waker's king of Hyrule/King of Red Lions
Favorite Link: Well, not much personality to judge here so I'll go with Breath of the Wild's Link, just because of his traversal skills and the fact you can drastically change his outfit (as opposed to just recoloring his tunic in some other games)
Favorite Impa: None of them really stand out to me but I guess Skyward Sword's?