What's you category for best? Long range - T5 AMC Close quarters - Shotgun (your choice just get one)
Many swear by the Lynx, but any automatic with grenade launcher plays to an engineer's role and abilities. My suggestion, the AV turret is the only must buy. That is by far the most effective weapon you can get as an engineer. Period.
med/long range rifle -- T5 AMC with HV ammo and Compen + Grips close quarters -- pump action shotgun or SMG (Hailstorm) with NV+SP Ammo and Supp.
Personally i really like the jaguar with adv. laser. accurate enough for (sort of) ranged engagements and lovely tight hip fire. I would leave the pump action for LA as you need to be almost within melee range to make the best use of it, and you're pretty much screwed if caught at medium or longer range (except with slugs, but they are too slow for anything moving usually) unless you can hightail it (eg jump jet) out of there. If you play many classes, remember that carbines unlock for your LA too (attachments included) and shotty for everyone bar infiltrator while SMG's are usable for all - this part really changes the weapons i select.
