2.75 inch all around. If you really want a combat load, then get 50 rounds or so of 3 inch 000. otherwise:
7.5 or 8 shot, that's your main practice load, too and 100 round boxes at discount stores are usually about $20. So I keep around 200 12ga rounds on hand. (400 20ga minimum, but the household has 2 20ga guns and 2 shooters)
#5, pheasant load (12ga, 40)
#4, pheasant load (20ga, 80)
4buck (12ga, 40 or so)
3buck (20ga, 100 or so)
00buck (12ga, 50ish)
slugger(tm) rifled slugs (12ga, 15)
brenneke heavy short mag (20ga, 15)
brenneke 3" 'silver slug' (20ga, 10)
I put all the info for the 20ga in because, IMO, it's a better choice as a one-gun answer. Availability of ammo in 20ga is just fine, and while it isn't usually any different in price from 12ga, I see no sense in increasing portion size at a restaurant 'just because'- same applies to shotguns.
I did a fair amount of research on the 20, and with brenneke slugs you are good on any game animals short of possibly moose out to 50-75 yards (depends on some stuff, like how accurate reports are). Anything in North America- including grizzly- will come down at under 25 yards, though with grizzly it's iffy, even with a 12ga brenneke hard cast. I only bring this up, because even if all you want is a bird gun, the grizzly bear in WSS is like Hitler in a political forum

Not terribly relevant, but alwasy becomes the main topic.
edit-
oh, and 20ga is noticeably less bulky to carry around!