The less choke, the better.
If you have a cylinder tube, I'd start with that. If not, go with skeet or IC. The theory is to have the least
amount of constriction present in the muzzle that might deflect the slug off target. But bear in mind that you're not dealing with MOA-level accuracy when you're shooting Foster slugs out of a smoothbore; rather, you're firing a substantially undersized cylinder of lead down a slightly oversized barrel. It's wobbling and rattling as it goes, and propellant gases are leaking out around it as it makes its way down the bore. It's not the most accurate projectile ever invented, and the last thing it needs is any significant choke constriction to smack into and knock it even further off course before it leaves the barrel.
The good news is that you can get decent accuracy--certainly tight enough to hit the vitals of a deer at 75 yards or so--out of some smoothbores, especially those with barrels designed specifically for shooting Foster slugs. The old Ithaca 37 smoothbore deer barrels, for example, are noted for quite good accuracy (i.e., groups of 4 inches or less at 100 yards) because they were purportedly bored tighter than bird barrels so that the slug was stabilized better before it left the muzzle.
Anyway, as with any slug-shooting endeavor, your best test is to buy a few boxes of different brands of slugs and head to the range and try out the various choke tubes you have available for your gun. You'll eventually come up with a combo that works, I'm sure. And if you find that you can't develop an adequately accurate load, you'll know that it's time to consider an upgrade to a fully rifled slug barrel.
Good luck. Also, you should check out the "Slug Shooting" board on this site; many guys there do nothing but ponder the possibilities of improving slug-shooting accuracy all day, and I'm sure they'd be eager to share their wisdom with you.