The evil counterpoint to that other thread....
1) Peter Pratt/Geoffrey Beevers
Creepy zombie Master. The Deadly Assassin is my favourite Master appearance. A being of pure hate and cunning that uses his intellect to almost tear Gallifrey apart.
2) Anthony Ainley
I'm referring mainly to his turn in Logopolis/Castrovalva but also his masterful performance in Survival. JNT made him ham it up in other stories but he could be a real, genuine threat if given the right story.
3) Roger Delgado
Love him, easily the most iconic. I think he's a bit too cuddly and cosey at times though and prefer the more menacing above incarnations.
4) Eric Roberts
Oh, wonderfully over the top and silly. The liquid snake thing was pretty cool too.
5) Derek Jacobi
Not on-screen nearly long enough but showed real danger and menace for the 4 minutes he was around. Would have liked to have seen more.
6) John Simm
Unlike most fans, I actually like
him in the Series 3 finale. Then we get End of Time and a Master plotline/performance that Simm has repeatedly expressed shame of in interviews. Still, I really liked him in Series 3, when he didn't have stupid CGI superpowers, so hopefully Moffat can rehabilitate him.