Heart of St. John Vianney
I'm not aware of any brains remaining incorrupt are you? Saint's bodies and body parts, such as St. Anthony's tongue, St. Catherine's hand, remain incorrupt - but I'm not aware of incorrupt brains specifically. That said, it seems to me the most extracted body part venerated as a relic has been the heart. Not the brain.*
*However, I did read somewhere that St. Gemma Galgani's brain remained incorrupt - But I can't verify it.