I didn't really care for Unova or Sinnoh. I don't think it was because I love the older gens more, I just didn't seem as impressed.
Kanto I think had a good design. Everything was real close to each other, but obstacles prevented you from getting into the newer stuff too soon. You could easily walk to any place and not have to rely on fly solely. That's something I appreciate.
Johto I thought was my favorite. It was a big design, the Sea route wasn't terrible. The Whirl Islands were impressive. I honestly don't have too many complaints. Although I felt they fell flat between Blackthorn and New Bark Town. I thought that route was eh.
Hoenn I think was amazing, although I wasn't a big fan of half of it being Water Routes...
Sinnoh I thought had its good moments.
Unova, I admit I haven't finished my Black 1 version, I lost interest in the game and stopped playing. (To clarify, I was raising the candle pokemon and got him up like 15 levels and my battery died before i could save. I got discouraged and gave up. I know i suck.)
As for pokemon diversity?
Kanto featured some of my favorites. The Nidoking/Queen line. I loved Grimer and Muk, i thought they looked cool. Pidgey is still my favorite looking starter bird. The Ghastly line was neat. Seal and Dewgong, Lapras, Tauros. A lot of my favorites came from this gen.
Johto was perfect I thought. It had all the old ones, plus just a touch of new stuff. Sentret, Heracross, Houndoom, Phanpy, Teddiursa were some of my favorites.
Hoenn was underrated for me when I first played it. I can't believe I thought that. I like Aggron, Absol, the starters were great, Slaking was neat. Plenty of diversity.
Sinnoh, the lack of fire pokemon was disappointing. The pokemon were decent though. I liked the Hippo one ( i forget how to spell it). I liked Lucario.
Unova... I liked Seismitoad. I thought it was cool looking. There were a couple others i liked, but i can't remember them.
I'm hoping that the new region is like Johto.