if someone plays "greater of 2 evils" on you during the recovery phase, what's to stop you from using your free recovery on your stunned character in response?

Your free recovery for the turn comes at a specific point in the recovery phase; it occurs during the wrap-up, after each player has passed on an empty chain and there is no more opportunity to play effects. If you want to recover a character earlier than that, such as after your opponent plays Greater of Two Evils, you'll need to play a recovery effect.

Incidentally, remember that Greater of Two Evils targets only a player, not a particular stunned character; to get off scot-free you'll want to recover all of your stunned characters.