Depends on what you're looking for.
If you want commentary from a person who knows a lot about the game, then Tasteless and Artosis take the cake. The might not have the flamboyant style of some other comentators, but they understand what's going on in the game and from that angle they are the best. They are the default comentators for the GSL for a reason.
Husky and HD are fun to listen to, but for the most part they just state the obvious. They are fun to listen to, but won't provide much info beyond what you could see for yourself by watching the replay with the sound turned off. This is especially evident with Husky, who often doesn't even see what's happening on his screen, much less has the ability to offer some insight into the flow of the game.
Day9 is... different. He's more focused on how the game works, offering advice for newer players. Also, his style is different from most other commentators and might take a bit of getting used to. But if you're looking to improve your game you can learn quite a bit from watching his casts.
Also worth mentioning is Psy, who is a good comentator with a solid knowledge of the game, but I rarely watch his casts since he mostly does his own games.