5th gen is my favorite. It has 2 starters I really like and a handfull of other designs I adored right away.
1st gen has my favorite starters, with all three of them striking my interes, and some of my old favorites, but nostalgia fans kind of bug me, plus it has the most filler designs.
3rd gen has some cool Pokemon, and decent starters, but is a little less memorable than some of the other gens IMO.
4th gen has some adorable monsters, as well as some pretty cool evolutions, but it also has my least favorite starters, as well as really dumb designs like Garchomp and Bastiodon.
2nd gen has a few cute and interesting designs, but SO many forgettable, uninteresting designs.