1)B2W2 Unova
2) Sinnoh
3) Hoenn
4) Kalos
5) Kanto
6) Johto
7) Alola
Unova and Sinnoh were great regions to explore, with the former taking it slightly. There were plenty of areas, and these regions were somewhat challenging and fun to explore, and were not linear.
Hoenn comes next, and despite having so much water, the tropical feel of the region made it a fun region to experience. Kalos was also not bad, though Lumiose was annoying to explore (though I give credit for it being Pokémon's first ever mega city). I especially loved everything from Lavere City onwards, such as route 15, Dendemille, and Anistar.
Then we come to Kanto, and yes, it can be pretty boring, but it's not dead last because it was the first ever region so I'm not being so harsh. In fact, while LGPE may annoy me in many ways, the one thing I'm really looking forward to is seeing Kanto again in HD.
Lastly, it's between Johto and Alola, with the former again edging out. Johto's main issue was that it was really small (infact the whole region can be covered without fly). Johto did have some nice scenery and places like national park, but overall, it just felt like Kanto was always meant to be the main appeal, even though Johto was the primary region for GSC and HGSS.
Dead last is Alola. Sad to say, this region is just not fun to explore and roam around. Each island feels so limited, and there's so many blocked areas in game till you clear certain requirements. Melemele Island has got to be the blandest place to explore in any Pokémon game, and Poni was just barren. Akala and Ula'ula were the only islands with some substance, but even then, the amount you could explore in these two islands is far less than previous regions.