General

any uncommon shiny. or great iv pokemon. most likely a combination of both, then again it depends on the rarity of the legendary that determines its worth.

Event pokemon with abilities exclusive to the event or IV and EV trained pokemon (like a 5 IV Jolly Garchomp for example). There are no non legendary pokemon that are so hard to get that people would give legendaries away from them.
That being said you could always swap trade as in trade then trade back.