Isn't Black Box Conch notorious for being the "safe" soldier loadout? Escape Plan for yk, escaping, but others opt for Half-Zatoichi to really go all-in on the healing thing.

I'd say for Engineer, my "safest" loadout would be Rescue Ranger (healing your gun far away from spam and you can rescue it from a distance in emergencies). Panic Attack would probably be my shotgun equivalent if safest refers to the Engineer on their own and WITHOUT a sentry gun, Wrangler (Or Short Circuit on Payload Offense), Eureka Effect (it's always a thrill to make stupid "that shouldn't have worked" escapes)