has to be either Federer or Nadal... The former brings it on key points nearly every time, the latter doesn't give up ever and fights until the end.
Novak has improved a lot, but has had a history of mental weakness with his retirements and other antics.
Murray still has a long way to go in this capacity and shouldn't even be in this discussion regardless of how highly he is ranked at the moment. There are many many players who are stronger than he is mentally, Ferrer, DelPotro, etc...
If we include past players Sampras would rank very highly. The guy was cool. Tres cool.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM