Gary Oldman
Nobody plays psychopaths better than he does. Spends most films practically eating the scenery and can turn a boring generic part in the dullest of films (Air Force One, Lost In Space) and will usually end up stealing the show. Also Nil By Mouth maybe the most realistic film ever about life on a British council estate.
Michael Caine
I don't really have a lot to add other than if his name is on the poster i'll watch it.
Seems just as comfortable appearing in a light comedy like Alfie as he is in an action film like Zulu or a bleak thriller like Get Carter.
Seeing him in a film is like putting on a comfortable well worn pair of trainers. And he's willing to help out new directors with smaller budgets get a step up the ladder as he showed with Harry Brown.... and he's still as good as ever.
Honorable mentions..
Robert Lindsay
David Morrissey
Robert Carlisle