I resoundingly nominate the one, the only Phil Hartman.
Also rans who who came thisclose: Bill Murray, Dan Aykroyd, John Belushi, Dana Carvey, Christopher Guest, Eddie Murphy, Will Ferrell.
Phil Hartman was the one that came to my mind upon reading the thread title, so Phil it is!
A toss-up between Hartman, Murphy and Gilda Radner. Radner’s portrayal of a drugged out rock star was worthy of the highest praise.
Gilda Radner, though Carvey was pretty good.
Martin Short. Never liked Hartman.
My vote is for Eddie Murphy.
Don Pardo.
I’d have to go with Eddie Murphy, he was SNL for several years.
-
Phil Hartman - The most painful SNL death, in my opinion.
-
Bill Murray, who has been way funnier outside the show, which perhaps shouldn’t count.
-
Eddie Murphy - Remember when he was funny?
-
Chris Farley - A very funny guy.
Charles Rocket
Chris Farley.
Hartman
Farley and Murphy. Not a lot of depth to their SNL work, but I never laughed harder.
Belushi. Period.
Belushi. There is nobody else even close.
Alec Baldwin 
I’d have to go with Gilda Radner. She’s the only one who still can consistently crack me up.
After her, Belushi and Hartman.
Belushi.
.
Beluchi
Hartman–by far the most versatile.
But my heart belongs to Ms. Tina Fey…
[though really as a writer and pin-up gal, not a performer per se]