So far I have Fast Track, Stealth, and Wheelman.
Fast Track is certainly good for leveling up faster, so a good first specialization.
I haven't tried Stealth, but I have been taken by surprise from people with Stealth.
Wheelman is amazing. The down time from being EMP'd is so small that you barely touch the ground in the Ghost before getting back up again. The buffed vehicle regeneration isn't very noticeable, and I honestly didn't know that vehicles did regenerate until I read the description for this package, but in Spartan Ops it becomes noticeable. The resistance to boarding isn't noticeable in Ghosts or Warthogs most of the time. The range and angle for boarding seems to be reduced, so on Warthogs and Ghosts you will only really see the reduced range, but Scorpions can only be boarded on certain locations, so you will really notice how hard it is to be boarded most of the time.