Just a word of caution. Most, if not all of these products are sourced out to the low bidder for manufacture now. Some of the companies have been sold outright to offshore owners, and are not the same quality as they were a short time ago. I have older cordless tools, but having compared my oldies to the same brand currently on the shelf, I find some corners have been cut. What I am trying to say is that if someone tells you that their BrandX tool is the greatest thing, if you go buy a new one today, you may not be getting the same quality of tool.
I can agree overall with everyone so far. Go Lithium, for sure. Just remember the discharge "curve" isn't so "curved". Those batteries give very little warning that they are nearing discharge. It's a discharge "cliff". Full power at the start of a cut or driving a screw, and unable to finish. Well, almost that bad. I use DeWalt pretty regularly that doesn't belong to me that is a new one (several, actually). It handles nicely, has a Li battery that has great power to weight ratio, but has noisy gears and a cheap chuck. Not the same as the older ones, IMO. Milwaukee has a good quality tool, but is no longer Domestic. The quality is not like it was. Panasonic builds good stuff in general, and their tools are no exception. I think they build their own batteries as well, unlike the others. I'd used, and seen worn out the Ryobi tools, and am not impressed, so they would not be high on my list. On it, but not at the top. Makita is good for tools. I worked with some electricians who would use nothing else, and they were terribly hard on their tools. The warranty was used quite a bit. I think the Makita tools stood up to the abuse rather well though.
No one has mentioned Porter-Cable. I will give them a thumbs up for overall quality. They would be high on my list.
I think there is a trend that points to Japanese made tools being superior to Chinese made tools. Just try to determine where the tool of your choice is made. It's nothing personal or political, but cost based bias that I think is going on.