as everyone else stated... depends on what you're working on.
I work on my jeeps and F250 truck, all 4wd. And occasionally my friend's jeeps/trucks. I use 1/2" for most things, and 1/4" for the rest. I find myself using 3/8" less and less. I don't know where the breaking point for 1/2" going up to 3/4 or 1" is, but for most passenger vehicles & trucks, 1/2" is probably sufficient. I imagine once you get larger than 1-ton vehicles, 1/2" might start to seem small.
Any 4wd vehicle I would say 1/2" is probably a good thing to have, especially for stuff like pinion nuts, suspension, steering. most things are larger on offload vehicles.