Well my current favourite Jedi is Revan. When I first saw the original SW movies my favourite was Luke; then after seeing the prequels it was Obi-Wan. Now after playing KOTOR, Revan has become my favourite mainly because I'm a sap for redemption and love stories.

Plus, Revan was probably the Jedi with the most potential of his era. He was a great duelist, probably one of the best(if not the best) of his era, a great military strategist and as Zhar put it: his 'potential was unmatched'. Revan's probably my favourite also due to fanboyism and last but not least due to the fact that I got the chance to play as him during K1. Oh and he's got those cool robes and mask!
Anyway...as for favourite Sith I'm not certain; I think Vader probably and mostly due to the tragic story he has. Exar Kun is cool too though.