Probably the SST by Hornady. Keep in mind while a slug, or bullet in this case may have the ballistics to work at 200 yards no company can claim accuracy at any distance. This is a pure function of the weapon it is being fired from. Hornady does have pretty good quality control though and makes good ammo for the most part. Probably some of the best in the industry in my opinion.

What is the best slug to use from your gun? Probably the one that works the best. Get a few and do some homework. Not all guns will like the same ammo, even if they are the same model. From what I have read though the 20 ga Ultras are fairly forgiving on ammo so it shouldn't be hard to find something that works well for you.

Also keep in mind everyones version of accurate, or "accurate enough" is different. My idea of accurate for a slug gun is 2 inches at 100 yards. Some are happy if they can group 6 inches. The only way you will know for sure is to get some ammo and shoot them. Please don't assume that just because someone on the internet says X brand of ammo works great in thier gun that it will in yours. You will need to shoot it to make sure.

Everyone suggested I try lightfields in my Ultra 12 GA, even the shop I got it from. My gun hates them! Go figure. Then again, they were accurate enough to hunt with, especially out to 75 yards or so. Just not the accuracy I was looking for.

Good luck, let us know what you find.

Paul