The new guy in our shop has a good basic set of hand tools and air tools, but asked what he should buy before spring so he'll be ready for service calls in the field. Three tools came quickly to mind. The others came down to personal preference.
1. A 4-foot-long, 3/4-inch-drive breaker bar. Mine is a one-piece Snap-on breaker bar, probably not available any more, but I've seen two-piece bars that accomplish the same goal: If compressed air for an impact wrench isn't available to break loose a stubborn nut or bolt, full-body weight applied to the end of a 4-foot long, 3/4-inch drive breaker bar will generally get the job done. Yes, a cheater pipe on the standard-length 1/2-drive breaker bar that everybody has in their toolbox may accomplish the goal, but---have you every been putting everything you've got on the end of a cheater pipe when a 1/2-drive socket or breaker bar breaks? At best it's uncomfortable, at worst it's dangerous.
2. A commercial-grade 1/2-inch drive battery-powered impact wrench. Service trucks often have compressed air systems available, but a battery-powered impact wrench is SO much handier and useful than dragging air hoses around, especially when you're working in the field. I hate to spend money, but lots of money is the only way to get a durable battery-powered wrench with every foot/pound of torque you can buy. Lithium-ion batteries are breath-takingly expensive, but they're significantly lighter than traditional batteries. Weight's not a big deal if you're just changing a flat tire, but if you're spending an afternoon changing field cultivator sweeps, an extra half-pound of weight makes a big difference.
3. A 1/4-inch drive socket set, in a self-contained box, with both metric and standard short and deep-well sockets. If the kit has a set of screwdriver, Phillips-head and Torx-head bits, that's even better. The goal is to have an all-in-one box of tools that you can carry into a tractor, sprayer or combine cab so you don't have to crawl in and out in search of the dozen or more different screws, bolts, nuts and other oddball fasterners that engineers love to use to hold seats, upholstery, consoles and displays together.
4.A big pry bar. Most mechanics have a set of pry bars, but I'm talking about a pry bar that takes two hands and a strong back to carry. At least 5 feet long, maybe more. And not a discount store, sale-of-the-week pry bar. Chances are good that at some point you're going to have every ounce of your body weight bouncing on the end of that bar---you want American steel that's not going to let you down. Literally.
5.A set of cheap open-end/closed-end wrenches. You can spend $1000 for a full set of professional-grade, 1 and 5/16 to 2 and 1/2-inch wrenches from a tool truck, but a decent set of wrenches from NAPA, Sears, or name-brand hardware store costing $100 to $200 will work fine. I don't use monster wrenches for field repairs on a daily basis, maybe once or twice a week during "the season," on large hydraulic fittings and major frame fasterners. In my experience, farmers don't need those monster wrenches often enough to justify spending big bucks, but on the occasions a farmer does need a mega-wrench, a name-brand, economy-grade wrench set is plenty-good.
I'm still mulling over this list. The newbie has a limited budget (don't we all?) and wants to get the tools that will help him most during field work. Now that I think about it, there are a few tools I need to add to MY inventory before spring work arrives...so many tools, so little time (and money.)