I would have to say, my favourite class in Pathfinder is the Sorcerer. For the benefit of the uninitiated, sorcerers differ from wizards in that they derive their spellcasting abilities from a bloodline they inherited from a magical ancestor such as a demon or dragon, rather than through scholarly pursuits. In game terms, this difference means they can cast more spells per day, but are more limited in the number of spells they know. They also receive certain magical abilities such as energy resistances or claw and bite attacks, but are hampered somewhat in their use of meta-magic feats (i.e. special abilities that allow spellcasters to alter certain aspects of spells such as duration, range, damage, etc.)
I play sorcerers a lot. In fact, I'd guess that roughly half the characters I've played, including my current character, have been sorcerers. One constant that applies to all the sorcerers I've built is that they have all been underpowered. The sorcerer class offers more options for character design than just about any other class and most of those options are bad from the perspective of optimization. Why? Because most of the bloodline powers encourage you to create a character that resembles a spellcasting rogue or melee fighter. Let me tell you, if you want to be a fighter in Pathfinder, be a fighter (or a paladin, maybe). Anything else is the very definition of suck in this game. Sure, it sounds cool to create a character who can launch a fireball, then charge in and lay a beatdown. Unfortunately, Pathfinder doesn't reward versatility. While the sorcerer is buffing himself, the fighter and the wizard have already killed every living thing within a half mile radius. High-level Pathfinder combat is a lot like the last two minutes of a football game. It seems to take forever, but in game time, it's only a couple of rounds at most. So to be effective, a character has to be able to contribute something substantial immediately and most sorcerer builds can't do that.
Notice, I said most. If you can pry yourself away from all the superficial coolness of the Abyssal or Draconic bloodlines and play to the strengths of the class, it is possible to be effective. First and foremost, the sorcerer is an arcane spellcaster and not surprisingly, the best bloodline for optimizing that role is Arcane. The bonus spells are all excellent, there's a nice selection of bonus feats and the Metamagic Adept bloodline power is quite handy when you really want to maximise that fireball and still run behind cover.
Sadly, I think I will always be attracted to the idea of a sorcerer who sprouts poison fangs and jumps into combat only to get promptly mauled because he has half the hp and AC of a real fighter. Like the scorpion who stings the toad, it's in my nature.