I feel like the obvious answer of Gen 5 is going to receive tons of love here, so I'm glad to see Gen 2 getting some for its sprites. I've only ever beaten Pokémon Crystal because of Phanpy, Teddiursa, and Skarmory in the same game, but it's really cool that there's a set of sprites for Pokémon Gold and another set of sprites for Silver. That said, I recently wanted to give G/S a shot with a very different team, and I kind of found it overwhelming to choose between the two sets of sprites.
That said, I gotta go to bat for Gen 3 sprites because they're the only ones not directly mentioned yet. Except for Gen 1. Gen 1 tried its best. I do prefer Gen 3's sprites over Pokémon Crystal's sprites (lack of animations in R/S and FR/LG aside). It's just how I think retro Pokémon games should look even though Gen 5 is probably way more pleasant to look at.
Both generations 2 and 3 offered a huge step up to the graphics, but there's something really cool about Gen 2 sprites even as someone who didn't fully play and appreciate Gen 2 until he was a teenager.