I don't think it'd be right to use an upper mid-carder heel for his retirement like Barret or at the moment Ryback.
Rey's been with the company for over a decade and I think losing to an upper mid-carder to end his career just wouldn't be right. If he's going to go out I think it should be to someone around his size and, with the exception of Daniel Bryan, to someone who's been with the company for years, a veteran like Jericho or RVD who would both have excellent chemistry in the ring.
Although it would be a stellar match, the only problem with him and Bryan would be that, if it were to be Wrestlemania 30, it would most probably be a championship match as I can't see Bryan without the title by then but I can't see Rey in the title picture.
Another person who hasn't been mentioned though is Ziggler. If Rey were to put anyone over I think it would have to be him. Although Ziggler is a bit bigger than him I still think they'd put on an awesome match