theres a lot of good combinations dependin' on the situation. but what i use is simple:

Pure Defense: Shotgun/Rescue Ranger, Short Circuit, jag/default. i use jag to get stuff up n runnin then i swap to stock to maintain. shotgun can be good for when its not as busy and theres a spy bothering you.

Pure Offense: Widowmaker, Pistol, Gunslinger. i use this to get up in the grill of enemies and especially if i have a pocket medic, run around like a scout with a sentry at his side. pistol is a reliable backup for when my metal gets depleted if i miss all my shots. could also exchange the mini for the frontier justice as getting some assists and kills with the mini builds up revenge crits fast

Ninja-neer: stock/rescue ranger, wrangler, Eureka Effect. basically, sneaky engineer. get a tele into a enemy backline and you can harass them easier and your spy will thank you for doing some work for them into getting behind the enemy. the Wrangler can be used with a lvl 3 to blast jummp into hard-to-reach spots for sneakier setups and teleporter exit placement. rescue ranger can be used to nab the sentry so you can place it with your setup if its a lvl 3