I'd say Henman - their matches have all had tiebreaks, and the three andy lost were a match with 2 TBs and the other two matches Andy had match points. It's that classic matchup I think.
Lleyton - classic power attacker vs. good returner/counterpuncher, I think their matches will always be close
Marat - their games just match up pretty well. I think if both of them are playing their absolute best, their matches could go either way
I'll think of some more
edit: Oh I see you made me choose, well thanks for allowing multiple options. I picked Tim, Marat, Lleyton, and David