Cactuars are the best way to level units, obtainable from raid summons, mog events, and the special cactuar dungeon in the vortex (only open 1 or 2 weekends per month). If none of those are available, the best exp per energy is full grinding explorations. I think Timber Tracks exploration is the best xp/energy ratio.
That said, if you're not 100% finished with story mode yet, prioritize that instead, as that unlocks espers, and a bunch of good equipment from quests and vault keys. Only specifically farm unit exp with your energy after the story (or if you simply can't beat storyline bosses with your current team).