I always take a look at what others are playing. I don't like stepping on people's toes, and greatly enjoy offering something unique to the party in regards to ability.
That said, I don't often have much in the way of issues finding a character that will slot in fairly smoothly. Lots of different roles are appealing in their own way.
I don't mind catering for a party in need but not because they lack a particular class because I believe you can never have too many of one class. For instance if you have a thief and he fails they the next thief might have success.
I admit having a preference to the more magical classes, whether arcane or divine, as well the more versatile or skillful classes.
Still to the question as I would pay attention to what other players are choosing for their characters, and wood take in consideration filling a role that has not been taken.
When it comes to the martial classes, my first choice is paladin or dusk blade/magus.
Admittedly, given the state of the class in Dungeons & Dragons and Pathfinder, I very rarely even look at the fighter simply for the fact it is so firmly locked in being mundane and being "Realistic" so this held back with little versatility or adaptability.
Some people are really good at cycling through classes as in being one class this adventure and a totally different one the next adventure but I can't do this unless I read and understand the mechanics of the character class in totality.