The difficulty in answering this forum is that generally speaking, only a few decepticons really get any character development. Unless you are a leader, your character just tends to follow the rank and file into battle, and occasionally give us a one line comment. That said, I will give you three wonderful characters for consideration:
1) Stalker from Last stand of the Wreckers. He had 3 of the wreckers' heaviest hitters at his mercy. He coud have just killed them, but found sick, villanous delight in torturing them.
2) Lio Kaiser from Transformers: Victory. What a bad @ss. He beat down everyone in his path for several episodes to include holding his own against Star Saber and Ginrai in a 2-on-1 handicap. He took out Road Caesar, Landcross, even Galaxy Shuttle. Most Con's only hold an advantage for part of an episode, sometime a 2 part episode. He was top bot for half a dozen episodes. Hell, it took a planet killing gun to finally stop him once and for all.
3) Overlord from Last Stand of the Wrekers. He killed a wrecker, put their leader on life support, ripped 1 in half, knocked out Kup, Perceptor, and that was just couple pages of book. He turned the last resort into a sadist playground, and why, to piss Megatron off (take that Starscream). He redefined villiany.