Freak

If you have to, pick two or three. Hell, separate into different sports, two or three for each. SWEET. For me:

NFL:
Brett Favre - Pretty easy for any GB fan growing up in the 1990s.
Reggie White - Same deal.
Mark Chmura - So he went on to assault some chick or something... who cares. TE for the Packers.

MLB:
Joe Carter - Game 6, 1993 World Series. History.
Roberto Alomar - Pre-spitting incident, the COOLEST Toronto sports star for quite some time.
John Olerud - I switch-hit in baseball; this guy is the only reason I ever tried Left. Sweetest stroke ever.
Juan Guzman - Second-coolest Toronto sports star in the early-mid-1990s.

NBA:
Mitch Ritchmond - I'm not sure why. He was awesome though.

NHL:
Nikolai Borchevsky - I still get mocked for talking about this guy too much.
Pavel Bure - Most exciting goalscorer of the 1990s.
Felix Potvin - THE CAT.