Old timers: Borg and McEnroe
Current Crop (in alpha order): the Bryans, Fed, Henman, Hewitt, Nadal, Santoro
Some choices are obvious; they're just fun to watch. Hewitt is included because his style is closest to my own playing style -- lots of defense but not afraid to come to net. Henman is there because that's the style I aspire to (and would explain why I was also a fan of Todd Martin and actively cheer Max Mirnyi in both singles and doubles).
On the horizon: Gasquet
On second thought, maybe Henman should go in the old timers group