OVO

It may sound unecessary, but I carry one of these and a DMM Pivot at all times. The OVO is bringing up the second, the DMM is for rapping and belaying the leader. 
I feel having the OVO for belaying the second is key for a few reasons:
- Very secure locking mode if the second climber weights the rope; no rope creep or slippage
- Handles a variety of rope diameters well, and is super smooth when taking in rope
- Lightweight, and lives on my back gear loop when not in use

All that being said, it is never my first choice to rap with. I have used a few tricks to help with the friction (two carabiners for the rope vs. one, etc.) but it is not where this device shines. 
For my money, it is the most simple, easy and effective device for bringing up one or two climbers securely and safely.