Who's the best blues harp player.

While Toots Thielman may have played some blues at times, I would consider him a jazz artist.

Little Walter haas always been one of my favorites for traditional blues.

I don't know about best, but Charlie Musslewhite is pretty good.

My votes would include those already listed above, and I'd add the following fine harp players to the list:
1. Carey Bell
2. Sonny Boy Williamson (I and II)
3. Slim Harpo
4. Kim Wilson (from Antone's in Austin, TX)
5. Paul deLay
6. William Clarke
7. Jerry Portnoy
8. Howlin' Wolf
9. Junior Wells
10. Jimmy Reed

Alot of really good ones have already been mentioned but Mark Wenner of the Nighthawks should be on the list as well. He's got one of those killer gravelly voices to boot.