Self Belaying: Some ascenders can also be used to protect a solo climber who is climbing along side a fixed rope. Unfortunately there is no simple answer as to what is the best device for self-toproping. Please see our advice page on Self-Belaying for more information.
Also, well worth reading is Petzl's page on Self-Belay in which you will notice that the Shunt, probably the most popular device used in the UK, is not recommended.
Finally, there is a review of several devices at Outdoor Gear Lab.