I'd say there are two options really.
1) Stone pets that be immediately helpful in your grinding the tamers/spirits/fabled. So pretty much any of the captured pets in the following thread. Capturing them as rares in a pet battle is better but not always easy.viewtopic.php?f=10&t=7556
2) Stone the pets that will have to be stoned eventually to get everything as rares (assuming that you want to work on that). There is a list at:viewtopic.php?f=2&t=2978