The best characters changed from season to season. Alot of the actors on the show I feel matured during its run as well so it's hard to say for definite. No one character was the best character consistently for ten years.
When friends started - Chandler > Joey > Phoebe > Rachel > Monica > Ross.
By the time had Friends had finished - Joey > Rachel > Ross > Chandler > Phoebe > Monica.
Joey was the most consistent for me, Ross and Rachel were the most improved, especially with the comedy but Chandler at his best (first six or seven seasons before the character became a bore) was better than any of the other characters at their best.
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