The biggest single monster I've run from a book was The Tarrasque, so CR 30.
Before that, there was a boss that used Contingency to change forms multiple times in succession (upon reaching 0hp, change into a new form). I don't know what the CR would have been if it had been statted out as a single monster, so I just awarded XP for each of the component parts (archmage, lich, ancient black dragon, kraken, kraken).