I am actually undecided between Mars and Saturn for my favorite planet. I like Mars since it is named after the Roman God of War. As for Saturn, I like that it has rings.
I've always been rather partial to Saturn, for possibly obvious reasons: the rings are fascinating and unique. I also like Neptune, I think maybe for its color (I like the blue...though for some reason, I'm not as big on Uranus even though it's a similar color).
Although I'm not too keen on trying to live on either of those planets. X) So I'll just sit here and continue to admire them as they are.