First unlock was for the ole SpecOps weapon, despite not playing as SpecOps much and, in truth, not even liking the class in general... My favourite class at the time was Support, yet I was informed that the SpecOps gun was one of the best unlocks, so gullable me went for it...
Now, not that I'm complaining or anything, but if I had any idea how much I'd turn out to love playing as Anti-Tank (And how scrumptious the DAO can be) I would have gone with that first... However, that was my FOURTH unlock, after the Engineer Jackhammer and Medic Rifle (In that order)... Then after the DAO came the Support PKM, followed (only recently) by the Sniper rifle unlock...
Seeing as Assault is my least favoured class, I have purposefully left it until last...