All solid games for me, but the ones I've played from best to worst is:
1. Ruby/Sapphire (Gen 3)
2. Fire Red/Leaf Green (Gen 3?)
3. Black/White; Black/White 2 (Gen 5)
4. Diamond/Pearl; Platinum (Gen 4)
5. Heart Gold/Soul Silver (Gen 4?)
6. X/Y (Gen 6)
7. Sun/Moon (Gen 7)
8. Red/Blue; Yellow (Gen 1)
9. Omega Ruby/ Alpha Sapphire (Gen 6?)