After watching "Dark Water"/"Death in Heaven", I have to say Michelle Gomez plays a good Master. Think the role suits her.
That said...I wish they had written her differently. To be honest...I find her a female John Simm. That same loony craziness. The Master has always had one problem throughout the show. One that the Doctor has always managed to avoid. Namely: having to follow the line set down by his/her predecessor.
Delgado was the first (and to my mind the best) Master. It is always fun to watch him. Plays the part with such relish. Can tell he enjoys it very much and the chemistry between him and the other actors is great.
Peter Pratt and Geoffrey Beevers only had one story each ("The Deadly Assassin" and "The Keeper of Traken" respectively), so it's hard to judge them. What I remember of Pratt is a well done performance. (Never actually seen "TKoT" to be honest".)
Next comes Anthony Ainley. Who is....good. Could have been great. Unfortunately it seems like most of his stories were written for Delgado's Master. There are moments where you can see just how good he could have been if given free reign with the character. Too bad he was never really given the chance. I think his last performance ("Survival") was his best. It is hard to judge between Delgado and Ainley because they are so similar. If they had been more different, they could have stood together (like each of the Doctor's incarnations do) instead of Ainley forever being in Delgado's shadow.
And next...Eric Roberts. Such a perfect casting ruined by a horrible, horrible script. If it had had a much better script, I think Roberts would have joined D. and A. among the best Masters. At the end of the TV movie, when he has become his full self. You can almost see how great he would have been. He has that charm of Delgado, and the psychopathy of John Simm.
Derek Jacobi would have been great if he had stayed the Master instead of immediately regenerating. Jacobi has even said he enjoyed it very much, would have loved to have come back. (Trivia time: This was actually his second time playing the Master. The first was in the "Scream of the Shalka" webcast. Which is on DVD now, I'd suggest picking it up. A very good story. Richard E. Grant is the Doctor...and honestly he feels like a cross between the Classic Doctors and the new. And kind of similar to Capaldi.)
And then came John Simm. Above I talked about the Master's regenerations not being allowed to be different. Well they did with this one...and how. I think it shows what can happen when you go too far the other way. Simm is a good actor. His role on DW is wacky, crazy, dangerous...but not the Master. At least, not to me. He has no purpose, except to torment the Doctor. And to me...that seems to miss the point of the role. Killing/tormenting the Doctor was never his main goal, it was more like a hobby. And I just never found his portrayal scary or menacing. I don't know, just never warmed up to him.
And finally Michelle Gomez...who feels like a female clone of Simm. Again, what's her motivation? The one she gave to the Doctor seems suspect to me. If that was truly her motivation...I think someone needs to reevaluate this character.
I think the problem with the Master in nuWho...is that the writers feel they have to PROVE the Master is a baddie. For instance her recent killing of Osgood to prove that she was evil. To me, that's just lazy writing. Plus it didn't have any impact because one, you saw it coming the moment the two of them were together and two, Osgood as a character is a non-entity.
I really think Ms. Gomez can be great in this role. But, the writers really need to give her depth. Not just have her capering around the cosmos cackling madly and twirling her figurative mustache.
