Personally I chose Operator upon achieving Rank 50 in Halo 4, but Operator may not be the best fitting for your your style of playing. This is a guide to what all Specializations are right for you by explaining all the details of each specialization.
Wetwork:
This class is mainly for assassain-style gameplay, AKA "Ninja mode" gameplay. Enjoy sneaking around like a boss.
The Tactical/Support package is Stealth. With it, you do not fully show up on Promethean Vision, (You only appear as an outline), Quiter footsteps, and you assassainate faster, leaving you less vulnerable.
Operator:
Vehicular-based gameplay, playin' with the big guns.
The T/S Package is Wheelman. Acquiring Wheelman lets your vehicle's health regen faster and reduces the duration of EMP.
Pioneer:
For ranking up faster and gaining as much XP as possible.
Package is Fast Track, in which gives you bonus XP (for as long as you had it on) at the end of a match.
Pathfinder:
Seeking out them enemies.
Package is Gunner, it reduces the overheat on turrets and removes the speed penalty on carrying detatched turrets.
Engineer:
Mechanically-based playing.
Drop Recon, you can see where an ordnance is dropping 12 seconds before anyone else, so you can
get those drops before anybody.
Stalker:
Spying on targets like a ninja.
Nemesis, which highlights the opponent who killed you previously. Get revenge every time.
Rogue:
For the lone wolves.
Stability will reduce your flinch when shot, increasing accuracy and making BR and DMR battles a breeze.
Tracker:
Seek out your opponent.
Requisition lets you choose a different set of drops for your ordnance by pressing up on the D-Pad.
--------------------
So which one best suits you? Leave a reply!