In my opinion, the best voiced out of the entire cast was Dr.Cid, though really I can't think of anyone whose voice acting wasn't satisfactory. In the case of Vaan, yes he did sound annoying but Vaan is an annoying character written in an annoying way, ergo his voice suited him. Larsa at times sounded off(most likely because his voice actor is not British, and occasionally struggled with the accent. It also sounded a little out of place, seeing as Vayne's accent was a lot more subdued).

So following the OP's original format I would say:

1. Balthier(Gideon Emery did a marvelous job in being very exspresive with his voice)
2. Vaan(because even though the annoying whine suits Vaan, his voice was definately the least desireable out of the main cast)

1.Dr.Cid(I could hear the relation to Balthier)
2.Larsa(just because of the occasional awkward accent mishap and the overall odd effect of having Larsa's accent be so thick, but Vayne's so subtle).