depends what you value - i.e. - what do you mean by 'bang?'
A Cooper, for example, is exceptional value if you're looking for a custom gun, at about half the price of what it would cost to have one made to spec.
If you're looking for the cheapest gun that goes 'bang,' period, look for an old beater Cooey. Accuracy, fit, finish, etc will be lacking, but they're generally reliable.
Or, if you're looking for reliable and accurate, but not necessarily pretty, then a lakefield/mossberg/savage mark 2 is hard to beat. Any old mossberg, generally, is going to be reliable, accurate and relatively cheap.
Or, if you must have a brand new gun, a Savage mk2 F would be my first choice, but I hear good things about Marlins.
Or, the cheapest of the 'premium' bolt guns, the ever popular CZ series. A step up from them would be things like the Browning A or T bolt, or the Ruger 77/22. Used options include Brno's, Remington 504's, Winchester 52's and repros, or possibly 69's.
Then there are the semi-autos, but I don't know much about them because most of the ones I've tried have been unsatisfactory for a variety of reasons. The buckmark carbine is the only semi .22 that I've tried that I actually like (and I've tried pretty much every semi that's currently available)