Unova > Sinnoh > Kanto > Johto = Hoenn
Unova (BW2) is the best, due to the great pokédex, a great region and a good team. The rival wasn't the best, but he had some semblance of personality unlike some of the others. It also had some of the best cities in any of the games. It also brought a lot of pretty cool designs. PWT and the battle subway were great on top of all that.
Sinnoh (Platinum) had arguably the best pokédex in any game. Sinnoh was a great region, and had the best team in any of the games. They actually had an idea of what they wanted to do, how they were going to do it, and actually worked towards it. The climb up the mountain to spear pillar and then the distortion world was awesome. New pokémon designs from gen IV were great too. Almost all of the new evolutionary lines were really good, and the evos to existing lines were generally good too. The rival was pretty cool on top of all that. The only problem I can really pick out is that the cities and routes felt pretty similar generally.
Kanto designs, objectively, are some of the best in the series. Other than that and nostalgia, it doesn't have a lot going for it though. Dull, linear region, and team Rocket never really did anything.
Johto was also dull, designs were slightly worse than Kanto and they didn't really make a lot of changes. Kanto tacked onto the end wasn't a great idea either, cause after the first time you do it it really isn't that exciting. Team Rocket also did fuck all.
Hoenn, had an incredibly dull pokédex. There were very few memorable designs, I think most forgettable pokémon are from Hoenn. A few were redeemed slightly when given evos in gen IV. The region was half water, which was boring. You either went 3 spaces and saw a Wingull / Tentacool or you repelled and just saw water with the occasional swimmer with a single water type that you'd probably just 1 hit. All in all, Hoenn was pretty dull and nothing really happened. I mean it rained a bit but that was about it.
