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My favorite is Unova because they're definitely pretty fleshed out. Also bringing back a Frontier Brain and making her an Elite Four and developing her at the same time was great.
I'm amazed at the Kanto Elite Four, they've appeared so many times now (especially Lance and Bruno) and they still don't feel that fleshed-out to me. I mean, FRLG and to a lesser extent HGSS have taken pains to show us more about the Kanto Elite Four and they are memorable just by virtue of being around the longest but I still feel that, for all the games they've appeared in, they should have received more exposure than they have. Even LGPE, which did a great job with Blue and Green's characters, didn't seem to do that much with the Elite Four.
In the Johto Elite Four, Will is practically a non-character and we don't know much about Karen either but at least she remains popular because of her memorable dialogue.
I wouldn't call the Hoenn or Kalos Elite Four great characters (doesn't help that half of them didn't even appear in the anime) but they've appeared more recently so they don't seem so bad. Give it another few years and they'll definitely seem more forgettable.
Sinnoh Elite Four was handled decently, all four acutally appeared in the anime and the games do an okay job packing in personality into the little focus they get.
It's just a bit of a letdown to me. The Elite Four are the strongest trainers in each region and many of them have barely been explored by the games and/or anime. On the other hand, the games generally do a good job of fleshing out the champions, so I don't know what it is.