callo,
Every shotgun is going to be pretty much a law unto itself as to what buckshot (or anything else) it patterns best. And it depends on what you think of as the best pattern for a defensive shotgun. Some folks want 'em wide open, some folks like tight patterns, otehrs want moderately open patterns.
But patterning isn't everything. Reliability is
Job One for a defensive shotgun- the ammo HAS to run in the gun. If it won't feed, fire, extract and eject reliably, how well it patterns is hardly relevant.
Your personal situation will also dictate to some degree what you use for defensive loads. If you have to be concerned with overpenetration, in an apartment or townhouse for example, you might want to use #4 buckshot, or even reduced recoil loads of 00 or #4 buck. If overpenetration isn't a concern, then you might want to use full velocity loads of 00.
I'm glad you didn't ask about the "best," because as is so often the case there is no real "best." But there are a lot of good options.
You can most likely select a pattern size by selecting the load likely to give you that pattern. If you want patterns as tight as possible, go with Hornady TAP FPD, either the red hull (full velocity) or blue hull (reduced recoil) version. The Federal Premium or LE loads that share the same FliteControl wad with the Hornady TAP will also likely pattern tightly.
For moderately open patterns, most any of the main line ammo companies' produucts will give you what you want. Federal's non-Premium or LE loads, Remington, Winchester, Fiocchi's nickle plated standard velocity or reduced recoil loads, etc. will likely give you good reliable moderately sized patterns.
If you want wide open patterns, try S&B (Sellier and Bellot) or Rio Royal, or any of the loads with soft lead, unplated pellets, no buffering or shot protection at all. Chances are those will give you real scattergun patterns, if that's what you want.
Cost I can't help with much, I'm afraid. Take a look at
http://www.ammunitiontogo.com/catalog1/index.php/cName/12-gauge-buckshot for some comparisons.
hth,
lpl