best choke for a 20ga. 1187 26" and heavy 13 #7
Any of the better manufactures in .555 - .570. I shoot the same gun with a 21" and I'm using a SSX getting 135-150 with most being on the lower end of that. If I were buying another I'd look at an Indian Creek or a Sumtoy. I've had really good luck with IC in a 12 gauge, they are just a pain to clean.
I have a truglo GSX .565,a Hastings .570,a Carlson.575 and a tightwad.570 and get killing patterns with all of them