'Rifled slugs' come in two flavors, Foster type and Brenneke. Foster slugs are loaded by most US ammo companies and are hollow based and usually soft lead. Brennekes are solid hard lead alloy with a screwed- on felt wad (original Brenneke) or have a plastic wad that is crimped into a recess in the base (Kent KO). Both have 'rifling' on the slug body that serves more to allow the slug to be fired thru a choke than to spin stabilize it, both depend on weight- forward balance to stabilize them in flight.
Rifled slugs are best fired thru smoothbore barrels, some choke (IC or Mod) is usually OK and in fact often helpful. Don't want to use Full in a fixed choke too much tho, or crank down a Polychoke tight or use a tight tube in a Cutts Compensator or equivalent. Sights help IMO. No substitute for testing and zeroing any given gun with different brands to see what it likes. Zeroing/bench testing with slugs is punishing, cheat as much as possible to help on recoil and keep sessions short. Whatever slug your gun shoots best and is zeroed for is the one you should use for all purposes, hunting and defense.
Sabot slugs work better in rifled barrels. Some use scopes and/or iron sights. Barrel lengths (either rifled or smoothbore) aren't terribly critical, 18-20" is enough for a full powder burn. Shoot whatever brand of sabot works best in your rifled barrel gun, note that sabots are generally a good bit more expensive than rifled slugs. Rifled barrels on shotguns play heck with buckshot patterns, you won't want a rifled barrel on a defensive gun if you want to shoot buckshot.
There is no "best" slug load, different guns shoot better with different loads. Find what your gun likes and buy a bunch of it. I have had good luck with the 2 3/4" Kent KO/Brenneke slugs in my smoothbores, I like 'em a lot and several of my guns do too. The fact that they are often on sale cheep!!! helps a lot too. Brennekes have a harder alloy of lead than most slugs, a sharp shoulder and penetrate deeply while cutting full diameter holes. Nothing not to like, long as you are careful where you unleash them so you are sure where they will stop. No need to buy the hype re. 3" magnum slugs, the short ones are OK.
hth,
lpl/nc (us old folks gotta stick together)