Top 5 is F2000, AEK, AS VAL, M60, M16A3, all "bc2 platinum". Out of those my favorite was AS VAL pre-patch unfortunately it now sucks since people were whining that it didn't have extended mags instead of learning to use it properly (it was a super accurate stealthy weapon).
Trail -- for the AT launcher one, easiest way is to use Javelin / laser-designated combo it counts. For the mines you can either passively put down 6 mines in clusters of two at the start of the round and 'set it and forget it' and change kits or if you want to proactively do it with a jeep you can load them and make sure you're driving really fast when you 'ghost' the vehicle into a moving tank early enough for you get out and you won't suicide. It is much harder than doing it with C4, just dont miss lol
Oh yeah for anyone going for the new assignments -- focus on getting ONE of them done asap, it will then unlock "Only for the dedicated" and start counting kills towards it. Learned this the hard way after getting almost all 4 new assignments done at the same time, so will take a bit longer to get the final one.
Also question for anyone who noticed, does Only for the dedicated assignment count any kills when using a kit, or do you need to use the kit specific weapons? i.e. if I use PDW with support, will it count towards "400 support kills"?