I've unlocked the DAO (Anti Tank), the G36C (Spec Ops), and the L8A51 (Medic).
That's the order I'd do it again, too. The L8A51 is just too unpredictable. It's really strange and difficult to get used to. When it works it's sweet, but too often I see errant shots that don't seem like they should be errant.
The G36C is nice and close and medium range, but I find that the scoped Spec Ops kits are more usable since Spec Ops are usually fighting from a distance. As a Spec Ops, I'd prefer to snipe an Assault solder rather than engage him at close/medium range.
The DAO, on the other hand, I use exclusively when using the AT class. The thing is badass at short range. At medium or long range, you run.