Although I use the wire wheel grinder the most, I'd say my absolute best tool that I use would be my head lamp. It's so nice having this little light right where you need it, and not having to use one of your hands to hold a flashlight. I have one that uses a band to hold the light to my head, and another one that clips to the brim of my hat. With the LED bulds, they light up the area well and last for a long time.
I do full time mechanic work, and i number one tool i have is my Mini Led Maglite. If i'm not using it, its in my pocket. The guys i work with have picked up on that fact to. You be surprised the number of times i hear "do you have your flash on you?" in a week
I Would have to say my most used tool is my 3/8 ratchet.There are so many adapters you can put on it.Swivels,torxs bits,allen sockets, Phillips,and flat head sockts,extensions.I'm sure I'm forgetting some.
There are a lot of tools I couldn't live without especially when working on my John Deere 140 but I gotta admit that the adjustable wrench is one of the best inventions. I know some of you can just look at a nut or bolt and know exactly what size it is but someone like me it is hard to guess and the adjustable eliminates multiple trips to the toolbox for me.
I just saw that either Snap-On tools or Mac-Tools just brought out a finger tip magnetic holder. It's made of rubber, looks like on of the fingers from a glove, but on the end of the finger tip, is a small magnetic that helps to pick up small screws or such in tight places. Might have to invest in one or two of them, as soon as I can find them.
It's not a tool, but it sure is a time savor. A plain old white sheet! I lay it down on the ground when working on a tractor, so if I happen to drop a bolt, washer or nut, it's easier to find. Especially if I'm working on a tractor in the backyard.