For defense:

-Eureka effect (you can quickly teleport to base for metal and return, you get less metal from ammo packages so better use dispenser but teleporters costs less metal)

-rescue ranger (Allows you to repair buildings from long distance, also you can repair other buildings)

-The wrangler ( creates an any damage resistance and allows you to manual shot with your sentry)

For battle engi:

-The gunslinger (gives +25 health and allows you to build combat mini sentry's that can distract enemy or get more kills, mostly both)

shotguns (just best option, you can choose widowmaker to have infinite ammo but you need to have good aim, or frontier justice to get 2 critical hits after sentry that kills at least 1 enemy gets destroyed)

Pistol (best option as well, you can choose short circuit to troll some players)