Fidget
The Destroyer
For plantera, just fly around her in a circle during her first phase and you're set. The seeds travel quite slowly so they're easy to dodge if you maintain a kind of rhythm. Second phase might merit the building of an OSOGOF tunnel (oh oh god oh ) that stretches up to the surface from deep below, because flying around her becomes so much harder.Hardest? Empress of Light
Most frustrating? Plantera.
Both are bullet hell but Empress of Light is at least fun and you can... y'know... see her projectiles? Meanwhile Plantera's seeds are barely visible and the thorn balls seem to just apparate. I think Empress is still harder for a few reasons but nonetheless "hardest" and "most frustrating" aren't the same.