Might be because of my main, but I enjoy watching Marth the most. I find it entertaining while at the same time I can pick up some new trick for my own game. And the differences between the playstyles of Mr. R and Mikeneko for instance are interesting to observe. The precision with the timing of the aerials, the options selected, the movement... Seeing a good Marth just fills me with joy.
Other than that, the all mighty flashy ZSS Salem sure is entertaining to watch, and so are many Warios, because of their movement in the air, the bike and the wheels and of course the Waft.
Strangely, I also kind of like to watch MKs playing MK dittos, since it's all just so fast and rather intense in my opinion.