It's a hard life being a recurring character on Lost. If you're not forgotten, violently killed or discarded, then life seems to be a series of merciless beatings and tertiary sidelining. Sometimes you can break out from the pack ( See Desmond or Ben, who were both awesome enough to earn regular status) But this is a list of the supporting players of this universe, to whom the show owes an unacknowledged debt either in terms of plot-leaning or quietly good acting. The qualification requires the character to have never been a regular up to the end of season five. Let's do this.
7) Penny Widmore
Sonya Walger did a very good job with this character, whose job it often was to appear in the last five minutes of a given episode and earn our emotional investment. Lost asked a lot of Penny, selling an epic romance while appearing so briefly is no small thing yet her relationship with Desmond has become the most genuine on the show. Fair Play
6) Tom A.K.A Mr Friendly
MC Gainey's job on this show was so often to stand in the background, maybe crack a one liner or two but otherwise do what he was told. Yet through the charm Gainey brought to the role, he ended up perhaps the most useful of the 'soldier' others. AKA the ones that got demolished once the show was done with them. But I remember this scene, and I remember that this character had my attention.
5) Danielle Rousseau
Before being unceremoniously capped in season 4, Rousseau did have her moments on Lost. And again, this is largely down to Mira Furlan's relatively subtle take on what is a fairly simple riff on the archetype of crazy bitch in the woods. Particularly in season two's Maternity Leave, where the character was given some real humanity, she set herself apart.
4) Charles Widmore
Alan Dale does play the same role in everything. Almost without exception. An old, bitter millionaire douche. Every time. First in the OC, then in Ugly Betty then in Lost. So the character's presence is more to do with what the writers have done with him more then what Dale brings to it, which isn't bad but is pretty stock really. But the cleverness with which the show tied Widmore into the island happenings, particularly in seasons 4 and 5, made him a major player, and that is set to continue in season 6.
A character whose continued presence in this show I think is solely down to the soul that L Scott Caldwell brought the character. Without her consistently turning in stellar performances, she no doubt would have been left in the dust. But she earned herself regular airtime and even some real story. She's never going to be anyone's favorite character, but she has survived Lost from the beginning whilst being entirely irrelevent to narrative happenings. Now that takes some chops right there.
2) Martin Keamy
Love this guy. On a show where everybody is always, always manipulating each other, something that can grow old very quickly, I can't tell you what a breath of fresh air it was to have a villain you could just love to hate, who would go around killing everyone in sight with minimal monologing like some 80's action movie villain (And it be a good thing). Like a stalling chess game was revitalized by a a strong, terrifically acted (Kevin Durand is pure awesome) warrior piece who has had it with the chicken shit games. Its a mixed metaphor, but what the hell, Keamy was exactly what this show needed when it needed it.
1) Richard Alpert
The rise of Richard Alpert from relatively minor other to fan favorite who became this mythical island figure whose very presence seems to lead to hysterical excitement that important shit is coming. Nestor Carbonell knocked this one out of the park form the beginning, lending a welcome calm and intelligence to the Lost universe. Which given the amount of stupidity our main characters often display, he is always, always welcome. He has even become a worthy third player in the Ben and Locke show, with his scenes with these characters having a bit more weight then certain other cast members.