I love the book ... but not sure I love the idea of trying to pick a favourite character.
In some respects Harold is a favourite, so well drawn by King (in the mini-series you miss a lot of the background and development of this character), and he's a character that got a raw deal by everyone (good and bad). A much more ambiguous character than most - except perhaps poor Nadine, who never really stood a chance.
Someone like Stu Redman is an easy pick for favourite, the obviously intended main hero of the story. Easy to like but he's not so interesting as many of the others (which is often the case with heroes).
The Larry Underwood is a wonderfully sleezy character that Kings paints and develops so well through the story.
Trashcan Man is fascinating but I find it hard to call him a favourite. Same goes for Lloyd Henreid, bad with brains, so rich a character in the book.
Nick Andros and Tom Cullen would come near the top of my list - particularly when I remember the interactions when it's just the pair of them.
Glen Bateman, the conscience of the good-guys, is an easy character to overlook (because who really wants a conscience?), but wonderfully well painted by King.
My favourite would come from that set, but I have no idea how to cut that back to one. ... I think I might go with what I think is the best written, the best developed (rather than most liked) character: Harold Lauder, he's such a wonderfully and easily dislikeable character, and yet you end up feeling sorry for him (well, I did). (ETA: But ask me again tomorrow and you might get a different answer.
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... I do find it interesting, looking over it, that those are all male characters - except my brief mention of Nadine. I can only say that none of the other female parts left such strong impressions. (I know Frannie, in particular, was probably supposed to, and early on she does, but her character sort of fades into the background as the story progresses.)
The Mother Abigail and Randall Flagg characters are, for me at least, not so much characters as they are story props: to keep everyone pointed in the desired direction.