1- Sinnoh: The best trio desig-wise, and it was the first time we had a trio that was completely dual-typed in the last stage evolutions.
2- Hoenn: These are probably the most popular ones, but I don't like things when they are too popular. Objectively, I like both their designs and their reliability in battles, especially their Mega forms.
3- Kanto: I like their designs (especially the Dragon Charizard and its Y Mega form). Battle-wise they are OK-ish, luckily they can Mega evolve.
4- Kalos: Only because of the awesome Greninja and because they are fully dual-typed in their 3rd stage evolutions.
5- Alola: Again, fully dual-typed is something that I love. I woulda loved 'em more if Litten evolved into something more gracious.
6- Unova: Man I LOVE Serperior, but Emboar and Samurott have some of the worst designs to my taste.
7- Johto: Their design is simple but at least they look better than Unova's. But the problem is that they are the worst starters for battles, no wonder, they are completely mono-typed and they have very poor movepools on top of it.