Is it possible to have a good Satan?
If we are talking about portraying the Father of Lies well, then actor Peter Stormare would certainly qualify as a "good Satan." In the fairly entertaining, though blatantly heretical movie Constantine, Stormare is rather effective in his brief screen time. This is the only performance I can recall where the basis for the character is insanity. Every actor sneaks in evil at one point or other though most try to make the character as human as possible. I guess this is to emphasize the dichotomy with evil. Al Pacino does this exceptionally well in The Devil's Advocate* as part of another stand-out rendition.
But insanity is a great choice. It just works. One moment he's congenial, the next he's a taunting child, and once he even seems to channel Brando. In between we see physical discomfort, personal conflicts, professional pride, and a wide range of emotions and physical behaviors. It's the kind of performance so convincing it scares.
*Just an aside, Keanu Reeves is the lead in both Constantine and The Devil's Advocate, struggling a bit at times to match the quality of his co-stars. In each of these movies and in the Matrix trilogy he plays a variation on Our Savior.