L85 is a waste of time, it sucks. Don't try and tell me otherwise. Do not get this.
G3 is good. However by choosing this you lose the support classes greatest attribute (nade launcher), so it's a waste of time.
PKM is an improvement on the other support weapons, however it still sucks. A good medic will beat a good 'supporter' any day. Waste of time.
Jackhammer is pretty decent for close range. However if you're Engineer, you should almost always be in a vehicle, so it's a waste of time selecting this weapon. For this reason, I didn't see it as an improvement over the other Engi guns.
G36c is an all round improvement. It's just better than the alternatives. The only problem being, I don't go Spec Ops, so this is a waste of time for me.
M95 has the unique ability of being able to shoot through glass n stuff. For this reason it's really cool

Sniping doesn't do much to help your team/score/whatever, but it's great fun. This is an improvement on the other rifles.
DAO12 OWNS. This should be your first choice. Get it.
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So...
If I were to start from scratch again, i'd unlock them in this order:
DAO
M95
G36c
Jackhammer
PKM
G3
L85