Like most, im starting from best to worst
1) Johto
Not the Gen II incarnation, which was dull and lame by todays standards (but for its time, it was absurdly awesome). It has incredibly interesting locations which are beautiful to look at, its maps are creative and stunning, its legendaries were few, but all of them were individual *cough*lake trio*cough*. The characters are somewhat realistic and not overly cartoon-ish, the rival is a total badass and jerk, LOVING IT! Its towns and gyms are memorable to say the least, and the gym leaders were good fun.
2)Sinnoh
Mostly for the same reasons as Johto, it has a really interesting and diverse landscape, generally memorable Gyms and Leaders, but a few towns are really forgettable, notably Celestic, Floaroma and Oreburgh. It loses out due to its annoying characters, the rival is insanely annoying, so much i printed a picture of him purely to burn it. It also has FAR too many Legendaries who are really bland and seem like filler than anything else, the lake trio are all the same, i have trouble recalling their names. Regigigas, YAWN. Heatran, i like him actually. and the dreadful Shaymin. Despite the wide variety of locations, not much is made out of them, they are really quite forgettable really.
3) Hoenn
I liked hoenn, it had a few nice locations, but because of their lack of real difference to everywhere else, they arent particularly memorable. There are a few notable gyms, locations and towns that are quite nice, but in general the region is really empty. Look at mauville city as the best example, its made out as rather high-tech and bustling, it's tiny and is about as bustling as a party hosted by me (I'm rather unpopular). It's a nice region, its just really boring at times, and lets not forget the half region of water!
4) Kanto
Bland. The routes are small pathways with little originality and seem to only exist as a small obstacle between even more boring cities. The only place thats even remotely interesting is Celadon, Saffron fails because of its blockiness. In Gen 1, where it was the begining, this wasn't too much of a problem, but Gen 3 and 4 remakes still fail to put some interesting quirks to this hellhole, although Gen 4 made a very good, honest attempt. The gyms were boring and even in the quirky puzzles of Gen 4, still has little challenge. Boring.