>>19852167This, pretty much. Systems wise XY is the pinnacle of the series, but the content is lacking. Gen 5 had a decent story if you're into that, and a fairly well-realized region with good characters. Gen 4 (Platinum since the other games in gen 4 were garbage, HGSS included) had a region that tried it's hardest to stay as non-linear as possible and rewarded exploration, and was challenging. Gen 3(Emerald) had a beautiful region with interesting Pokemon, a story that felt like it mattered but didn't really care all that much about making you into the main focal point unlike BW, had extremely catchy music, and ran like a dream on the GBA. Gen 2 was a perfect sequel to Gen 1, but failed to really stand out. Gen 1 started everything, but really shouldn't be looked at as an example of the modern games' design. Colosseum/Gale of Darkness tried something different and while they were THAT great, were compelling in their own right, even if Gale of Darkness felt like a step back.
For me, not counting balance hacks like VoltWhite and Sacred Gold, my favorites in order are Emerald, Platinum, XY, BW, Gold(nostalgia), Colosseum, Red/FR/LG, BW2, Gale of Darkness, and HGSS bringing up the rear. Keep in mind I've played since Red way back in the olden days.