Ryobi, most cost effective, IMO. I have the 18V impact driver - like others have said, it drives 3" screws easily using one hand with no stripping. I also have a host of 18 volt Ryobi tools, none of which have failed me. I own and use the following Ryobi +1 tools:
Impact driver, should have bought one years ago.
Leaf blower, not the most powerful but blows those leaves off the deck every morning with no hassle and the wife loves it because it doesn't weigh anything.
Insecticide sprayer, I love it because it holds one gallon, doesn't weigh much and will spray those pesky weeds all over the property and I don't have to pump it.
Small flashlight, handy anytime, usually comes in a kit.
Large flashlight, somewhere around 1 million candle power, not sure but using the Lithium Ion batts, it doesn't go bad after one season like the $20 versions do and goes for around 40 minutes before discharge.
Small reciprocating branch pruner, works good, long blade.
Reciprocating pole pruner with extension, goes out to around six feet and is easy to grab for those simple branch cutting jobs when I don't want to drag out the pole chain saw.
Vacuum cleaner with all the attachments, used for cleaning the vehicles and can used as a blower.
Radio, used all the time when working out in the yard.
Jig saw, nice quality, don't even use or own a 120 volt version anymore. Used to build and repair my decks.
Circular saw, much easier than dragging out the corded version, which I gave to my son anyway.
Angle grinder, no match for the 120V version but it is real handy for those quick, one-off jobs.
12V drill/driver. I pre-drill holes often before I screw things together so it help to have a smaller tool loaded with a drill bit on hand.
Chain saw, for those times when I just need to saw a 6 or 8 inch diameter branch and don't want to fire up the gas saw. Also keep it on hand to extricate my gas saw when it gets stuck in a log.
Weed eater, the wife uses this one and loves it because it very light. I stick to the gas-powered Echo but I like the larger battery that came with the Ryobi.
Sawzall, one of the handiest tools, ever.