RBY: Classic designs, but some of them aren't that good. Dragonite is ugly as hell. The gym leaders are awesome, and they were able to get away with designs they wouldn't do now, like Lt. Surge.
GSC: Not as good as RBY in Pokemon design. Sure everyone remembers Wobbuffet, but come on, Slugma and Quilfish? Shit. The gym leaders all look good, but the final four are pretty forgettable. Battling the old gym leaders and the champion from the last game was one of the greatest ideas ever, what idiot decided to abandon this idea?
RSE: Too many been there, did that. Two evolutionary stage bug lines? Real original. The original gym leaders were okay, but the Battle Frontier leaders were far more cool looking. This has the worst music ever, the leader/chamption battle themes are crap, though the Battle Frontier battle theme is win.
DP: Grass/Ice? Electric/Ghost? Grass/Ground? Oh fuck yes, these are the best designs ever. Leaders look great, with Shirona being the best design in the whole series.