I would say the full circuit of Isle Of Mull from the Craignure. Take the road towards Iona, take the B8035 all the way round to the B8074 which you follows over to Calgary, Dervaig and Tobermory. From there, the ferry from Tobermory or the coastal road down to Fishnish or Craignure again.
Alternative, and I have done this route myself, is the road from Craignure to Salen, the B8035 to the crossroad to B8074 and then follow this road over Calgary and Dervaig to Tobermory. It is an absolute fantastic ride with some medium to big hills on single track roads. Dervaig to Tobermory with the twin peaks was a killer.
This ride is in my top five of my best ever rides.