Starscream

In all honesty, few people give Starscream the credit he deserves. Sure he may seem like a whiny pansy, but he's just a really great actor.
Starscream has ambition in plenty, which is part of the reason why he puts on the weak-willed act. He follows Megatron without question, except when his ambition gets the better of him and he makes his move prematurely, which, unfortunately, happens a lot.
As far as Starscream's history, there are many different stories. In the G1 series(that's Generation One for you newbies), Starscream began as a scientist partnered with Skyfire, who I'll tell you about later. He enjoyed his work as a scientist, but when he joined the Decepticons and got his first taste of warfare, he couldn't get enough of it.
In other stories, Starscream was the Sky Commander in charge of Trypticon Station. He turned Decepticon when Megatron broke into the station and managed to control Dark Energon, a previously uncontrollable source of energy. Starscream was in awe of Megatron and his ability to use the Dark Energon. He still had his ambition though, and constantly tried to take Megatron's place.
Starscream ranks as the Decepticon's number one flyer. In jet mode, he is the fastest and the most skilled. While Megatron knows that Starscream has his eyes on his position, he keeps him around because he feels that they are similar and believes that he would act the same way were their roles reversed.
To be honest, the Decepticons would be much better off with Starscream as their leader. Megatron is all about brute force. Starscream, on the other hand, knows all about brute force and how to use it, but prefers stealth, style and subtlety. And where Megatron is almost always focused on vengeance, Starscream keeps his mind on what the Decepticons are all about: universal domination.
I guess it's a pretty good thing that he isn't in charge ;) Anyway, if you take a good look at what Starscream is all about, you'll soon see why he's my favorite Decepticon.
Bumblebee ;)