Hot Coldman
Banned
This has already been addressed above, but RTD's rather crass old example is still the best one - if he's killed instantly, he's dead. If he gets shot in the head, that's it, game over. We now actually have an example of that, I suppose, in The Impossible Astronaut. Even if faked.
That's also why when he gets shot by the Dalek in The Stolen Earth it's a glancing shot, as a dead-on shot would've killed him dead.
I think the key thing is that the regeneration process isn't interrupted or impeded. and that means don't fuck with the brain. Massive blunt trauma (such as Cass' ship crashing) would probably destroy the brain, making it impossible to trigger regeneration, as would suffocation. A shot to the head could actually be survived if the bullet managed to miss the brain, I reckon. Plus things like the Dalek lasers or River Song's weapon don't seem to be localised, so frankly I could see the Doctor surviving a single shot from a Dalek every single time, glancing or not. Of course, the Dalek can always fire again.