The beginning and the end of Johto are great, it's the middle (Johto League Champions and the start of Master Quest) that brings the show to a screeching halt in terms of plot progression. The league is great, and Misty leaving is definitely something you should watch if you grew up with the original trio.
Hoenn starts off as bad as mid-Johto, but once Drew is introduced, May's contests become much more entertaining (which is relieving because Ash doesn't accomplish much during his stay in Hoenn), and the Battle Frontier is great, although you might want to find a Japanese sub for it due to being the first season with a completely different voice actors.
Sinnoh suffers from slow plot progression that isn't much better than mid-Johto, abysmal rivals compared to Kanto and Hoenn, and what might be the Scrappy-Doo of the Pokemon series sharing the spotlight with Pikachu, but the villains are the the best they've ever been and the league was great if you discount how it ended. Can't recommend you specific season for this one unfortunately, as the good is pretty dispersed among the bad,
They didn't make an anime adaptation for Black and White, if anybody tells you otherwise they're lying to you.
