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Kanto - 3/5; basic but gets things done well enough. Not much more to say. Same goes for FR/LG
Jhoto (I'm assuming not combined) - 2/5, much like Kanto but lacking in stuff so it loses a point. Unless we're talking about HG/SS, then it's 3/5
Hoenn - 5/5; great arrangement, diversity, and hidden/optional locations. Does the tropical theme really well. A little bit too much HMs, but dive is my favorite HM in the series to date. I cannot comment on OR/AS because I didn't play it, I refuse to give money to a hoenn remake without the frontier or something as good/better replacing it.
Sinnoh - 1/5; holy shit fuck the main game areas. Extremely bland areas for the most part, the only unique areas (marsh and snow) are bogged down by stopping/slowing down to a crawl. Not a 0/5 because the post game areas are legit. Horrible HM usage.
Unova - 2/5; also bland much like sinnoh but is saved by no stupid slow down shit and shitty HMs. Not a 1/5 because seasons are pretty cool.
Kalos - 4/5; I think people just clump this region as being shit only because it's gen 6. I don't like gen 6 myself, but the region is great. It has plenty of diversity and great arrangement (route 1 length aside), I enjoyed going through it. Only thing that would make it a 5/5 was more secret stuff.
Alola - ?/5; seems great to me so far on the premise that it's like Hoenn, but I can't tell for sure without playing. I predict it's at least 3/5.