I like Tom Baker a lot, but for some reason I just prefer Jon Pertwee.
I think it's because the Third Doctor had a kind of maverick streak which I felt suited the character very well.
Davison was average. [Colin] Baker was heinous. Sylvester McCoy was actually a slight recovery (back to average), but not nearly as good as the first 4 Doctors (exempting Troughton - I've not seen any of his so can't judge).
Unfortunately, McCoy pulled some of the worst stories ever written for Dr. Who...
Ecclestone's first few episodes (regarding his portrayal of the Doctor) were just above average, but then he started to 'get into' the role and was, I think, the best since Baker in the second half or 2/3rds of his season. It's a shame he quit when he did, I think he would've done a far better job than Tennant.
Tennant's not been bad. I just don't think he's been especially good. The closest parallel to Tennant is, I feel, McCoy (though I'd still place McCoy above him). Average, is okay when given a good story, but is mostly rather underwhelming.