I think overall, as a series, the sprites from Pokemon Ranger are by far the best out of every game. When I first picked up the game, I was amazed by the detail put into each sprite, they all look great.
However, I've also always been a fan of the Gold sprites. I love their simple beauty and the fact that they can be so detailed, yet only use 4 colours. Sure, some look tacky and disproportionate, but the style itself has a certain charm that attracts me.