Favorite: Eliwood's promotion and Chrom's promotion were pretty much tied for me, because they both get a good new weapon type, and the promotion bonuses are nice. Eliwood's got him a horse though, and Chrom's got him two really good skills. Eliwood's was kinda late though in his mode (not so much if you use the first one in Hector's mode), and Chrom's could be done as soon as you got a Master Seal, which the earliest guaranteed one was at Ch.8 I think. So Chrom does have a bit more going for him, but this is a poll on most and least favorite ones, so I chose Eliwood in the end because that horse helps him quite a bit.
Least: Leaf's promotion due to it being really bleh, and very little to him, besides more levels. I'd fix it in that he gains more stats from it, and he gets some kind of cool skill from it. I do like how he still gets EXP like an unpromoted unit though (if I remember correctly, but I may be wrong though).