I'd love to see Charlie Sheen make a return to television and in particular Two and A Half Men. Friends of mine who have worked with him in the States all proclaim he's an ego to himself but his comedic style and timing is impeccable. The chemistry between the cast really worked and had me in stitches constantly. Please bring him back - I'm positive ratings will increase ten-fold.
Unfortunately I never really warmed to Ashton Kutcher being on the show.
I've just finished working on a comedy series and a sitcom pilot and it become quite obvious which actors have their comedy chops and which don't. There are some actors that have an innate sense of beat and timing when it comes to comedy. Charlie Sheen is one of them.