I think this question would be awesome for the "English Teachers" group.
I look at Orsino as a typical love-lorn Shakespeare character~one who will continue to pursue his love interest despite the shutting doors. He is not very sympathetic to Olivia's mourning, this put him in my "selfish" character column. I think he enjoys the pursuit, not unlike the high school boy who constantly wants to date the popular girl in school when all he has to offer her is his love because he has not shown her any other traits except desperation.
I adore Maria and her flirtatiousness, although her taste in men is a bit suspect. Toby isn't buying into Olivia's mourning household either and that, makes him a selfish character. But, he is a lot of fun and when this play is seen, not read, Toby is usually a bit of an Animal House frat boy. Maria's vindictivenes is a bit harsh; I believe the trick played on Malvolio is very overthetop if viewed as a reaction to his anger at the party atmosphere. I believe Shakespeare lays enough clues to hint that Malvolio was this difficult even before the death of Olivia's brother so Maria takes a stand and gives him a hard time of it.
Viola is my favorite character since she is not a weak female; she takes control of her destiny even when mourning her brother, Sebastian. I don't agree with her choice in men, Orsino being a bit of a wimp but she is the one who takes action in their impending relationship.