For someone new (like me) I would agree & say first see Death Rides A Horse & then Django (after the man with no name trilogy these are top 2 for me) then...
Ace High (ELi Wallach, in the same realm as Good Bad & Ugly)
Per 100,000 Dollar Ti Ammazzo AKA Vengeance Is Mine (very underrated)
Sabata (Lee Van Cleef, say no more)
My Name Is Nobody (Henry Fonda is great, get to know Terrence Hill's comdic style
Navajo Joe (once you get over that it's Burt Reynolds...)
Companeros (revolution, Thomas Millian, Franco Nero, Jack Pallance & mayhem)
Great Silence (Klauss Kinsky, cold hearted killing & lots of snow!)
God Forgives, I Don't (violent & gritty, try to get the widescreen version)
& Korano's list...especially
Blindman (a blind gunslinger with great aim & better one liners)
Let us know what you think & be honest, even if it's an unpopular opinion.