In descending order, my top 5 candidates:
(5) 109G6 and G10. Shaped like a dagger, a dagger that's going to be plunged into your heart if you aren't careful. Dark and foreboding camoflage, eerily evil in design. You can add and subtract parts to build the perfect monster. Dr. Frankenstein would be proud.
(4) C-205. Tend to be seen on the AKDesert map than any other map, probably because of the camo pattern is perfect for the terrain. Like a snake, it'll bite you if you get too close and step on it.
(3) FW-190A8. Because it is dark and packs four 20mm cannons. Its angular canopy exudes evil. Has all the elegance of a broadsword. When it comes straight at you, be afraid. Be very afraid.
(2) LA-7. Crude design, big nose, 3 cannons, high speed. A peasant's weapon, an engine of death. You can hear it coming a mile away, and it doesn't care because you can't run from it. It will chase you down and it will kill you.
And the Number One Evil Plane....
(1) F4U-1 Corsair. Dark and shaped like a bat, it flies like Dracula at Halloween. The heavy canopy framing accentuates the evilness of this bat-plane. Spawned from Hades and flown by demons; one, two, its after you!