Probably squirrels because it's the closest to "real" hunting I can get. A nice accurate rifle, a stand of hardwoods, and I'm can forget that I'm not somewhere in the mountains stalking the wiley wonderbeast. Plus I can literally walk out back and be hunting in 50 steps.
I love to deer hunt, but around here that's mostly done with dogs. A fun way to do it, and I really enjoy it, but it takes a lot of people, land and dogs. Not something you can do on your own.
This year will be my first try deer hunting without dogs. They've been raiding the garden and harassing me when I'm squirrel hunting, so when the season opens, I'll walk out back, find a nice spot to set on the hillside and watch the creek bottom. If nothing else it gave me an excuse to buy a couple of rifles I wanted.