The best tennis coaches were usually mediocre players. Like Brad Gilbert, Darrin Cahill, and Tony Roche. Maybe because being a crap player makes you think strategically to beat better, more talented guys.
I'm gonna go with Murray, who's pretty mediocre as a player. Not sure if he's smart enough to be a good coach but who said Brad Gilbert was especially smart. Sometimes dumb players just need a dumb guy who has already been there. Out of your choices I'm gonna go with Sexy Stepanek, he's the definition of a mediocre player who outwits his opponents and wins ugly (literally