First a few things
1) this should be a multiple choice poll
2) You should list some parameters as to what you are referring to when you say best tang "I.E. best algae eater, least aggressive, size etc etc"
With that being said, I voted for a Gem tang as in my experience with them in customers tanks they are not as aggressive as other "round" tangs like purple's, sailfins, yellow's etc etc, not to mention they are great algae eaters AND are absolutely BEAUTIFUL!!!
I wish I could afford a tank large enough to keep a trio of Gem tangs, a trio of Black tangs as well as a Blonde naso a Sohal tang and a yellow belly hippo tang