Like many here, I have a tough time definitively answering that question. But I'll try to go for an honest ranking of how I feel at this moment.
1. Patrick Troughton. Probably the best actor of the bunch. He's got such a keen intelligence to him. He's always doing
something even if he's not the focus of the scene. He seems to be having the most fun. And he's the only Doctor with whom I could really buy him being put in jeopardy in an end of episode cliffhanger.
2. Tom Baker. No explanation needed. He just absolutely owns the role. Even though I ranked Troughton above him, whenever I think of the Doctor, I first think of Tom Baker.
3. Christopher Eccleston. A ...

... FANTASTIC (There! I said it!) actor who brings real gravitas to the role just when it needed it most. He's a goofball but with some nice dark edges that fit perfectly with his new backstory. And while I was really bummed that we didn't get more of him, I suppose it's admirable that he wanted to go out on top. Of his 13 episodes, there's not a single one that I would rate below a "B." He's the only Doctor that I can say that about. He's not perfect though. I didn't like the romantic elements of his relationship with Rose and I didn't like it when he would start referring to humans as "stupid apes" or acting like a dick to Mickey for no good reason.
4. David Tennant. Sadly, he's one that has seriously gone down in my estimation over the last year or so. (He used to be #1.) At times, I just love his manic energy and generally friendly persona. But there have been too many instances in his run of them just running the character into the ground with loud overacting, self-righteous pontificating, deifying the character, and just plain crappy stories. It's been the writing more than the acting but still, I suspect a different actor wouldn't get quite so bogged down and would find things to do with the material besides "Faster, more intense."
5. Sylvester McCoy. I really like the energy he brings to the role. He would rank higher if more of his stories were as good as "Rememberance of the Daleks." More than any of the others, it's good characterization sabotaged by bad writing.
6. William Hartnell. He's a difficult one to judge because his role on the series was so different from any of the others. The other Doctors are clearly the lead on their show. Hartnell was more of an enigmatic, eccentric instigator while Ian & Barbara did all the dramatic heavy lifting. But there's a great chemistry to that original cast and Hartnell was a big part of it.
7. Colin Baker. Once you get past the costume and get accustomed to the crankiness and remember that he was saddled with crap companions, he's very good. He's got a theatricality that I think the role demands.
8. Jon Pertwee. He's got a lot of natural charisma but he's too serious to be the Doctor.
9. Peter Davison. He's a good actor & a nice guy but he's too sedate and not eccentric enough.
10. Paul McGann. Too little information to go on. Tough break dude. I at least hope that FOX will get off their asses and put his 1996 movie on DVD in R1!