Nadal- when he's in the zone you get crazy inside out FH's, down-the-line FH's, killer angle BH's (usually when least expected), touch volleys, solid slice, some of the best defensive skills ever, some of the best offensive skills (aggressive serve to set up FH winner on next shot.) The only thing you don't get is tons of aces, which is fine by me because aces do nothing to excite me.
Federer- Graceful as hell, can hit every shot in the book (almost.) Though aces bore me, his spot serving in pressure is amazing.
Stan- as a 1HBH guy myself, my jaw just drops when his BH is really clicking. That alone is worth the price of admission. When the serve and FH are also on, he's like a freight train that can't be stopped.
Dolgopolov- Like Monfils, Kyrgios etc., he often does very ill-advised things, but when he's on they land in and are usually spectacular. Unfortunately, these days are pretty rare.
DelPotro- that huge forehand, nuff said. Hope he comes back.
Tommy Haas was always fun to watch when he was on. I got to watch him practice at Indian Wells and even that was cool.
Anyways those are some (not all) of mine.
On the WTA side, I love watching Cibulkova, Halep, C. Garcia, Giorgi, Lisicki, and of course Serena, on their good days.