Well, this thing has moved fast.
Down to the finals...
Hartman vs Ferrell
Phil Hartman was great. Neither of them was nearly as funny as Eddie Murphy.
Ferrell was on the show longer than Murphy (7 years > 5), and had as many or more "funniest guy on Earth" moments as Murphy. Only difference is, a lot of Murphy's pinnacle moments came outside SNL, but Ferrell's fame during his run was almost all SNL-based. I guess his first "big" role would be Mugatu in Zoolander (2001), but his first HUGE role would be Frank The Tank in Old School, a year after he left SNL. So I think Ferrell > Murphy is a pretty easy call.
Hartman and Murphy were totally different players. Different roles to fill, but they both filled them well. This isn't about who is funniest. It's "Best SNL Cast Member", and you can take that to mean a few things. Is it the guy who was a superstar for 5 years and then left (Murphy), or is it the guy who held the show together for a decade, merging the Nealy-Carvey-Meyers cast with the Sandler-Spade-Farley cast, never being a superstar but always lifting sketches with his presence (Hartman)? I think it's a coinflip, but I voted Hartman.