(I don't think HG/SS count as merit for the IV Gen, just like FR/GL don't count as Gen III, in my opinion)
1. Gen II - Best game. Full of great content and great pokemon designs (They didn't overdo it).
2. Gen I - Nostalgia might have something to do with this, but it's definitelly top 3 no matter what. Not all designs were perfect (looking at you hitmonchan), but they were original and consistent
3. Gen III - Great pokemon and interesting enough story. Up to this point everything seemed to come together perfectly with the exception of the beauty contests.. Great starters.
4. Gen IV - Some great pokemon too (Mostly new evolutions from old ones), BUT this is when they started over doing it a little with a few of the designs. Legendaries officially became Digimon and gundams.
5. Gen V - Here's when it started to go south for the regular beasts.. There were some great pokemon, but they started to look more like Digimon.. There was a drop in simplicity in favor of "over-the-topness". starters were a disaster.
6. Gen VI - More Digimon and ridiculous designs but some good mons were included (as always). Starters were a disaster.
7. Gen VII - Weirdest pokemon ever.. Alola forms are interesting but half the beasts seem to be legendary digimons (There's an over excercised moskito man that looks like the evil monster from a power rangers episode for god's sake..). Worst starters ever created.