Sorcerer.
It was a great mechanical addition, when it was introduced in 3E. The 5E changes to the way Wizard slots work really takes the wind out of that niche, though. Also, the more they try to add flavor to the class, the more it becomes apparent that what mechanics do exist are extremely mismatched to that flavor. The Sorcerer mechanics look more like someone with magical OCD than someone with magic coursing through their veins.
This became especially apparent, to me, during the conversations around the new Psion (er.. Mystic). What I've always used psionics for is to represent people who, for one reason or another, have some inherent magic. It didn't work quite like a caster. There are no words, gestures, or materials; you make the magic happen by internal power and force of will. That's what fits the flavor of the Sorcerer. Using the same casting mechanics as a Wizard who needs to manipulate "the Weave" makes absolutely no sense.
There's not necessarily anything wrong with the shell of the class. It would just serve much better as a framework to hang things like Warmage, Beguiler, etc. on than Wild Mage, Dragon-blooded, and Favored Soul. If you want to include those, and do so using the existing spell list, rather than inventing an entirely new mechanic, there is a way to do it.
The Warlock provides a much more unique casting framework. It is clearly designed to feel different. Pull the Sorcerer subclasses out and tweak them for Warlock. then add a "Pact of the Blood" to represent those born to power, rather than having struck a bargain. That's what I've done for my game. A Blood Pact Dragon-blood is just peachy balanced against anything else at the table and it feels a lot more interesting, compared to the Wizard than she did as a Sorcerer.
Lots of people are saying Warlock, because you don't need both it and the Sorcerer. I agree, but I think their solution is backwards. Kill the Sorcerer, keep the Warlock.