Personally, if you are going to use Engineer a lot, Jackhammer is a pretty good choice. I use MP7 quite a lot so if you are Engineer player, which I cannot see that many people to be, then go for this so that you can get MP7.
However, even though the GL is not as good as before, it still has some use to it, also F2000 has some great fire power. It is just that you need to work on the aiming since it is not the most accurate gun that is availiable out there.
MG36 is a good gun too but I don't usually use it since I use PKM more often.
I thought P90 would be pretty good but P90 has some serious RoF (Rate of Fire) and stopping power issue. It doesn't have enough power to take down the target fast but you could always spray and pray. DAO-12 is my personal choice here.