I find it hard to beat Ryobi for battery compatibility and bang for the buck.
The 18v system has one battery terminal and all the batteries are compatible. So the oldest 18v blue tool and the newest hi vis yellow-green tool work from the same battery.
Bang for the buck is also very high with Ryobi. HD has two kits on sale right now at the 2 and 3 hundred dollar price point, as well as a buy two batteries & get a tool promo. Black Friday seems to be less of a good than Ryobi days, but still pretty a good deal. For instance, during Ryobi days the two battery kit cost $99 and came with a mostly similar list of free tools. Right now it is $129, down from $149 at the beginning of the week.
Ryobi has a very big system, geared toward homeowners with a few forays into professional/trade tools. I have probably 20 Ryobi tools and 20 Ryobi batteries, and I've had one failed battery (one cell died) and one failed tool (trigger switch failed on a grinder). I wish parts availability were better for the tools, but at the price point it is usually just as cheap to replace as repair.
If I were doing it again I would go with Milwaukee for shop and Festool for home, but that is because I'm a prosumer who cares less about bang for the buck. I also have to go 20 minutes for a HomeDepot but my local independent hardware shop is both a Milwaukee and Festool vendor.
