Theres not enough customization available to worry about a dedicated "best build" like in a WoW. If youre playing engineer you just have to kinda know what kind of player you are up front. DO you prefer the shot guns or SMGs? If you like to crash objectives
you should take the Field computer or if you hang back take the UAV. For the other items I would take the slugs when available, and skip the UCAV and the decoy. The UCAV is crazy weak and can't spot enemies and the decoy grenade means you have to sacrifice
Although you could take the decoy, and swap them out in-game as needed....hmmmm. Anyways hope this helped a bit.
Let me tell you about the Decoy- Unless you are playing "Decoy" this is probably the best tectical piece of equipment. Not only do you get a kill assist from every team kill on affected players (Adding points to team score) But the decoy also looks like
an enemy soldier taking cover- when used correctly, this can be the single most imortant piece of tech for a game mode like siege or stockade. But your success with it is dependant on how and when you use it- of course. Its effects also last 2.5x longer than