I once heard Cleese speak about the 3 main characters (Basil, Sybil, & Manuel) and about audience perceptions of them. It seems that the audiences had the most sympathy for Manuel, and the least for Sybil, because Manuel is perceived as being the most powerless, while Sybil as the most powerful. Basil, of course, is in the middle, and probably the most hapless, and therefore the funniest.
Cleese also mentioned that an inspiration for his character was the typical Brit stiff-upper-lip, Great-Chain-of-Being middle class male, who yearns for a strong class structure but can't rise out of his own class, and who tries desperately to sublimate an intense rage. That's certainly one of Cleese's specialties--the barely supressed rage of a put-upon male who believes in fair play.