Shimano

Ultegra 6700-A 10-Speed Rear Derailleur Pulley Set, ver.2

These are original to my M591 rear derailleur and they work well with it when the old one's needed replacing after 3000 miles. They last plenty, I might have gotten away with simply servicing the old ones but their teeth were noticeably worn enough to justify replacement, and the replacements are smooth and shift better.