Its true that a lot of Wire characters had development but Bubbles is definitely a huge standout and really the heart of the show. Its true that a lot of folks like Greggs and Freeman had some development but they still largely stayed the same people from beginning to end.
But another one that stands out to me but probably gets forgotten is Carver. Its a side story but seeing him go from being Herc's comic relief running buddy to a real cop who cares about the community is a story I really enjoyed the second time I watched the show. Its especially revealing when played against Herc who stays the same dumbass making mistake after mistake but falling backwards into more personal success each season. Carver stays a grunt cop (with stripes or without) fighting the good fight but becoming a better one with each season. S4 Daniels recommending him to Prez as a good cop and them having the "Carver?" "He's changed. He's come a long way." conversation is a good example of that, I think. Of course Carver foolishly tries to help Herc and trust him and it ends up biting Carver in the ass, getting Marlo's crew off the hook, and screwing Randy. And of course Carver carries the weight of that mistake while Herc falls backwards into the profitable law firm gig.
Those are the two standouts to me on the show. Lots of guys fall, rise, or stumble but those are the two ones I like watching for 5 seasons grow as characters. McNulty would fall in there for the first 4 seasons if he didn't just fly off the reservation in S5.
