I'm not a Will fan either... but he's HUGE.. so I'm the one missing out... I just don't see what it is. Chris Farley was kind of like that for me as well... oh he DID make me laugh, occasionally, but in general I didn't get Chris either.
Great comedic actors are actually rare. Comedians exist in abundance... but 'acting' funny, in a movie, and IMHO, is a rare talent.
No such thing as 'greatest ever'... but for me the top ten would have to include: Peter Sellers - Charlie Chaplain - Gene Wilder - Laurel and Hardy - every single member of Monty Python.