Ukraine

I'd think you'd struggle to get even his immediate, hopefully liberal, successors to give it up. There are even more liberal Russians that support Crimea's annexation on account of it historically and ethnically being seen as Russian, obviously the ethnic cleansing practiced during Stalin's time helped ensure this. The Crimeans themselves are hard to say because nearly any poll that has actually been conducted on them has been biased one way or the other. I'd still guesstimate that a good majority of them will possibly want to stay with Russia since war support seems decently high there. I don't think there is an easy path for Crimea to come back to Ukraine even if the Russians agree to relinquish it.