Well, in the new series (2005 onwards) I'm betting that it was Tennant who killed/destroyed the most enemies onscreen(Fried all the cyberman's brains making them die in agony on alternate earth, millions of Daleks and cyberman he had cast into the void, whilst eccleston hesitated to kill a single Dalek, sure he did "destroy them all in the war", but it was Rose who had gained omnipotent powers and erased them from (that current) existence.
Doctor who's resurrectionformula, that was a coincidence(the original actor who played the first doctor was in very poor health and died a few months later), and they had him "regenerate". The tardis also regenerates, so you can have a new modern setting and a new lead actor, who changes companions from time to time so the show never "ages". The limit of the regenerations is 13, according to the 1996 movie, but in the new series The Master claimed he was resurrected(after he was thrown in the time vortex and died) on Gallifrey and given a new set of regenerations(actually in the original series he had cheated "death" more then once. Since the access to Gallifrey was destroyed by the master and the doctor(the end of time). "The fall of the 11th", is announced, so likely they'll still have two more doctors after Smith departs, and then you could conclude the series if ratings were to decline, or find some way to let him live on such as a new regeneration cycle.
Merlin for example started in 2008, and is a real success, both shows also share the best family timeslot, early saturdayevening just after dinner.