1 - Roger Delgado - Affably evil is a good description of him and his mannerisms. He is a cross between a gentlemen, a scholar and a thug, rather like Moriarty and a match for the 3rd Doctor. His scenes with the Doctor are almost always standout, whether it's fighting one another or being tolerant of the others existence.
Of course he has his problems, like the fact that he's almost always double crossed.
2 - John Simm - One of the problems with RTD's 10th Doctor was that he was just as flippant with evil villains as Douglas Adams 4th Doctor, which had the habbit of killing a lot of tension. As such John Simms Master makes a great foil for the 10th Doctor, being equally as flippant towards the "heroes". He is pretty much a dark reflection of Tennants Doctor.
Also for all the goofiness of Last of the Time Lords, Simms Master is perhaps the most effective of all the Masters.
Well, until the whole "I believe in fairies" thing.
3 - Decaying Master - The Grim Reaper Master. A bit comic booky but when the look was done right he actually looked to be the scariest villain/monster in Doctor Who.
I get the feeling RTD tried to copy that idea when it came to "End of Time", problem was it just ended up being utterly silly.
4 - Anthony Ainley - This is a hard one, Ainley's Master had a lot of poor stories and hamming it up didn't help. I feel he was at his best in Logopolis, Five Doctors and Survival, but the rest damn poor.
5 - Derek Jacobi - If they ever do bring the Master back, I hope he's like Derek Jacobi's the very short stint. Chilling, such a shame we didn't get to see more of him.
6 - Eric Roberts - Not bad, but the venom spitting thing just didn't work for him.