I have Makita 18, Dewalt 20, and Milwaukee 12. I had a few onesy twosy units from other brands over the years starting with a small ni-cad hand drill.
I could talk about the pros and cons of each for hours. Instead, I'd like to pick a few winners and the battery paradox.
Early Milwaukee 12V batteries were garbage. They typically failed in less than a year. New batteries are great. The M12 system is small, compact, and handy as hell. You cannot beat the convenience of the drill and driver combo set for screwing things together or kit assembly jobs like Ikea furniture, Purple Martin condo assembly, etc etc drill the hole with the drill, tighten the screw with that driver. The driver is small enough to outclass a power screw driver. I like this combo sooooo much that I gave a set to each of my 4 kids for Christmas one year.
Dewalt 20V tools are the bomb. They are big and heavy, but they are amazing. The big 1/2 driver generates massive torque. My big air gun has gathered dust ever since I got it. Big huge tractor lug nuts get done with ease. And the 3" gang nuts on my disc plows are handled with aplomb. I even bought the small compressor and I use it for inflating tires from 8psi in the big tractor tires to 80 psi on the planter gauge wheel. So far it beats bringing the machine to the shop. It is slow compared to shop air, but it has an auto set. You just set it and walk away to do something else. The other Dewalt goldmine is their 20V chain saw. I have his and hers machines (a long story). My big husky commercial gas chain saw gathers dust for all but the biggest trees. For most 20V tool needs, the Dewalts are equal to 115V corded tools. I have not had the need for a big 60V Dewalt tool so I can't comment. That in itself tells you how good the 20V system is. The 20V Dewalt are my favorites today. I have friends who swear by the Milwaukee system too. I prolly have 10 Dewalts that all work great so I'm not about to switch. Besides, 3 different batteries is already too many.