I like Sheridan in the second season. It's amazing that over ten years later, I can still him as Tron. But in the few years after that, he really starts aging fast.
You are right on about G'Kar. But I think the opposite happened with Londo. For good or bad, he is set on fulfilling his destiny, which is kind of boring. For me, Vir is the real stand-out for the Centauri in S2.