Best Lens for Portraits

Hey guy's,

I have a question , What is the best lens for portrait photography ?
( Body Canon 7D )

Single portraits and group shots.

environments :

-Studio
-Outdoor
-Clubs
-Party's

Is there one lens for all of these environments ? or a specific lens for a specific environment ? in that case which lens for what environment ?

Thanx
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© Helio Phoeli