Uhmmm I guess anyone in God mode would be fun to watch as that means they're producing awesome tennis. Federer, Nalbandian, Safin, Wawrinka, Del Potro and Gonzalez are all amazing to watch at their best. Federer and Nalbandian for their beautiful, varied and all-court game, Safin and Wawrinka for their stunning baseline power-tennis from both wings that seemingly never breaks down despite the risk, and Del Potro for his amazing fast and Gonzalez for his even more ridiculously amazing forehand. As already mentioned, when Tsonga hit absolute God-mode in his semi against Rafa at the AO'08, it was just a sight to behold.
I'm pretty sure God-mode Sampras must have been amazing as well.
On the women's side, Serena and Venus, Kvitova, Henin and Clijsters for fairly recent examples. Mauresmo's God mode was beautiful too, she really has an all-court game that was fantastic to watch especially in much of 2006. That said, there are very few current female players who seem to have a God mode. Could you imagine someone like Wozniacki entering something called a God mode? What does that even entail, her being able to finish off forehand sitters for once?
Basically, it comes down to offensive and aggressive players. Their God modes are naturally the most fun as their shotmaking would be in full flight. Djokovic and Rafa playing in God mode are very impressive too, but it's different since they aren't the kind of players that will smash winners all over the place. Djokovic had a God mode match against Ferrer once, and that was amazing to watch as well. Rafa was basically his own kind of God mode in spring/summer 2008, but it's not the typical shotmaking kind that stands out among the others I've mentioned.
