I dislike leaving any tool behind when I leave the shop for field repairs, because the tool left behind is inevitably the tool needed for a specific repair. But there are at least five tools that if I forget them, I will turn around and go back to the shop to get them. They include:
-a 4-1/2 foot long, 3/4-inch drive Snap-on breaker bar. That particular model is no longer available, and I guard it like a child. There have been times on service calls when my 1/2-inch drive air gun wouldn't break loose a bolt, and my 1-inch drive impact wrench wouldn't fit in that particular tight spot, where the mega-breaker bar saved the day. A 3-foot long cheater pipe and a torch may have been involved, but all that matters is I eventually got the bolt broke loose.
-a boxed set of 1/4-inch drive sockets, in both metric and standard sizes. If I'm climbing into a cab or another area where I know there are lots of small fasteners, it saves a lot of steps to have all the relevant sizes at hand in a convenient carrying case.
-battery-powered impact wrenches, with appropriate swivel-type impact sockets. I'm to the point where I'd just as soon go home as work on equipment without battery-powered wrenches. And I'm nearly as fussy about using swivel-type impact sockets. They allow you to reach and get onto nuts and bolts without having the impact wrench perfectly inline with the fastener. Rather than duplicate all the standard, "straight" sockets I already have, I bought swivel sockets in only the sizes I use the most: 10-, 13-, 15-, 18-, and 24 mm. I've not regretted that miserly strategy.
-my electrical diagnostics box. I took a portable plastic toolbox and put my multi-meter, my butane-powered soldering iron, my crimping tools and all the other stuff I use for electrical diagnostics in one easy-to-carry box. It's loaded with heat-shrink butt connectors in a half dozen sizes, heat shrink tubing, electrical tape, and all sorts of stuff related to electrical repairs.
-my full set of screwdriver-handled pry bars, ranging in length from 12-inches up to 6-feet long. Once you get used to using pry bars to lift, shift, twist or otherwise move things on equipment, it's way better than lifting, shoving and tugging with brute strength. If I could figure out a way to carry my pry bars on my belt like I carry my pliers in a leather pouch, I think I'd do it. I kind of like the idea of whisking a 3-foot pry bar out of a sheath on my belt like a swordsman unsheathing a sword, but…we won't go into my fantasy world.