12ga pump is the most reasonably priced ....and with the variety of shells available today from 1 1/8 of shot ...to 1 oz ...or even 7/8 oz they're the most versatile ...
Make sure you buy a gun that has screw in changeable choke tubes...again so its versatile...for clays, for live birds, for defense, etc....
Ideally the best barrel length for a "clays gun" in a pump or semi-auto is a 28" barrel ...and it may not be optimal for a "fighting shotgun" ...but its fine for general defense.
A gun with a 3" chamber is fine...( so it can shoot 2 3/4" or 3" shells ) again so you have flexibility.
Mossberg, Remington pump guns offer a good value ....
a better pump gun in my view is the Benelli Nova series...
A much better pump gun, in my view, is the Browning BPS Hunter model in a 12ga and 28" barrel...its a gun you'll have for a long time...they list new for $660 but you'll find them between $500 and $600 new in box. Its a really solid gun....smooth action, safety is on top of the tang -easy to reach even with gloves on...ejects out of the bottom ...stock is cast neutral so it doesn't favor a right handed shooter over a leftie...
http://www.browning.com/products/cat...id=012&tid=211