A new booklet, Y Pellennig: The Remotest Hills of Wales, is attempting to answer that very question.
Topping the list in Wales is West Tump, which is a 17m (55ft) high rock out in the Celtic Sea.
On mainland Wales, the 467m (1,532ft) high Tyle Garw in the Brecon Beacons is 4.9km (3 miles) from the nearest road.
Qualification to be listed among Y Pellennig - which translates from Welsh as far or outlying - requires the hills to be a minimum of 2.5km (1.5 miles) from summit to the nearest road and a minimum of 15m (50ft) of drop from the summit to the lowest point connecting the hill to next higher ground.
The booklet, by Myrddyn Phillips and Aled Williams, provides a hill list that will take the user to some of the most beautiful land in Wales.
Mr Phillips said: "It is a unique concept and challenge as no grouping of hills have been listed by remoteness and no known person has visited all the summits.
"Each hill name has been painstakingly researched by Aled Williams taking in all available Ordnance Survey maps, old estate survey maps, nautical charts and many local enquiries.
"Many of the islands listed are out of bounds during the summer due to seabird nesting colonies, or their general inaccessibility."
The full list and all the details of Y Pellennig are now available online.
There are few better places to get away from it all than in the hills of Wales, but which summit is the most remote for solitude seekers?