Obviously it depends on which you prefer the look of but just with respect to the mod you unlock at lvl 10 of the specialization, the first one i recommend is pioneer with the fast track mod. This grants 20% extra experience when used which will help you progress a little faster through future specializations.
Do some research on which weapon skin and armor as well as emblems each unlocks as this may sway you ( i had to have the wetwork helmet ). It ultimately depends what you like the look of and what your playstyle is. See : http://halo.wikia.com/wiki/Specializations
All of them are garbage unless you're into weapon skins and armor. Otherwise, Operator gets you wheelmen which let's you recover faster from EMP's in a vehicle while also coming with the coveted pokeball emblem. Another "decent" one is Rogue for the no flinching when being shot at while scoped.