I think there are various ways to tackle this question. Probably no one should follow in my footsteps. I've got:
- Dewalt 60v
- Dewalt 20v
- Dewalt 12v
- Porter Cable 20v
- Black & Decker 20v
- Ryobi 18v
- Bosch 18v
- Bosch 12v
- Milwaukee 12v
- Worx 20v
- Ridgid 18v
- Craftsman 60v
Basically I'm not loyal to anything. I have little reservation about adding yet another platform if they have a tool I want or are the best deal.
If a person wanted to have only ONE brand, I'm starting to think Ryobi could be the frontrunner (see what I did there?). With the HP tools, Ryobi is dipping into tools that can be considered pro-grade. The tools are usually cheap. Most important of all, they are legion. If you're after something neat an innovative, Ryobi probably has it.
That said, most of my tools are either Dewalt or Bosch, so I'm not even taking my own advice.
Having so many platforms can be a challenge. I built a wooden toolbox-style holder that holds most of the chargers I need all together though. E.g. A single Dewalt charger handles the 60v, 20v and 12v batteries. My Bosch charger has slots for 18v or 12v. Ryobi and Ridged are mounted externally to the ends, I don't actually use Black & Decker batteries - my Porter Cable batteries slide right into the tools.
That leaves Worx (I only have one tool currently, the powerwasher, anyway), Craftsman v60 (I don't need to pack my OPE batteries around often) and Milwaukee 12v. I would like to add the M12 charger to my toolbox kit, but I ran out of room... maybe a signal I've reached the end?
In many ways I think three platforms would make the most sense: 1) to be your main pro-grade option (e.g. Milwaukee or Dewalt), 2) Ryobi (so you don't feel left out when something cool comes out), and 3) M12 - because small tools are handy.