Maybe I'm a little biased (Silver was my first Pokemon game), but second generation is really the best. At times, RBY's gameplay is clunky and slow. Whereas RSE piles on too much all at once. And GSC introduced some of my all-time favorites. Wobbuffet, Gligar, Octillery, Sunflora, Flaaffy, Feraligatr, Quilava, Politoed, Porygon2, Sneasel, Qwilfish, and Quagsire!
There's no real bottlenecks in gameplay like in RBY or RSE. Everything flows.