Gen V was probably my favourite generation for many reasons. I really liked both sets of protagonists; it's hard to choose which pair I preferred. I think in terms of design I prefer the originals, Hilda and Hilbert. (Though allow me to lead with my absolute heartbreak at their official names). In general, I just like that they seem to be older than any other pair of protagonists throughout the Pokémon series. The games that came before Black and White were HeartGold and SoulSilver, and while Gold and Silver used to be my favourite games, I felt the protagonists were.. 'younged-down', along with the Rival. This also applies to Black 2 and White 2 - Nate and Rosa (and Hugh) generally just feel a little younger than Hilda and Hilbert, and it's pretty much that reason alone that makes me prefer the latter. I would say that BW's protagonists were about 15/16 in Black and White, and B2W2's protagonists were about 12/13/14 (at any rate, younger) in B2W2.
In terms of design, while I've never played as the female in Gen V, I prefer their design to the males. I really don't like that Nate's colour scheme is so similar to Hilbert's - seriously, hair, hat, jacket, same colour. They're what I normally take into account when looking at a protagonist, so while I liked Hilbert's design a lot, I was very disappointed with Nate's being so similar. I do like that the B2W2 protagonists' designs showed their hair - I suppose Game Freak ran out of ideas for creative headwear, so they resorted to simply showing off their absolutely crazy hairstyles, which really paid off in my opinion.