The last shoot I went to I shot Rio 7.5's. Several people were shooting my 1187 20g and it got 500+ rds through it and the gun cycled great with the Rio. They are clean and have good rims for good ejection.
If I couldn't have Rio I'd shoot Winchester AA's. I gave up cheap shotshells a long time ago. Inconsistent patterns, dirty and ejection/cycling problems.
I buy the Rio by the case at a local shop for $5 a box.
Cheap shells can choke a pump as well.