I can chime in on this one since all I do is watch streams:
1. Tom Brady - Undisputed best
2. Aris - Combination of knowledge, humor, personality makes him #2. Even if a boring match is on Aris will make it entertaining
3. Pig - I feel like I become a better player just absorbing the knowledge and experience that he offers up, very level headed style
4. 16 Bit - Hype commentary, accurate knowledge, can be pretty funny/entertaining. Might stray off topic at length on geeky subject matter if nothing hype is happening on stream
5. Arturo - Solid knowledge and also entertaining. Enthusiasm for Injustice comes across to the spectators and makes it very enjoyable
6. Medina - Haven't heard him in a while but always remember being hyped/entertained when he was running an MK9 stream
7. Reo - Don't hear him too often (usually because he's involved in the top 4 or so) but always makes the stream fun and enjoyable when he's talking
8. Dogysamich - Does he still commentate BnB? I haven't caught a full BnB in a while but it seems its usually two random tourney players filling in inbetween their matches, but having Dogy there as a constant was always nice
These are just the picks from the ones I've heard. I may have left some good commentators off the list that I just haven't heard, like GamerBlake for example.