As far as I understand, it doesn't matter what kind of stunts you do, the amount is always the same. Right?

What's the fastest animation for a stunt? Which specific stunt combo should I use?

to my understanding, all stunts are roughly the same length. the game encourages mixing up ur stunts due to trick repeat penalties as is, so i say learn a couple (2-3 is a safe number!) to start with, make em muscle memory, and gradually add more the more you play ^^

throwing in flips or spins also helps avoid repeat penalties!

okay, ive done some actual testing now, and from what i can gather, there is no one Ultimate Stunt, however there are a couple stunts which combine the fastest moves from each tier. generally speaking, any stunt starting with Rollover (r1 + <-) or Heel Clicker (r1 + ->) and ends with a Buttspin or Holy Roller are among the fastest to perform in my experience