I thought it was going to be good, but this site still took my breath away (and I don't mean from the climb up to it) and made an instant jump into my "top 10" (dolmens, I can't judge site types against each other).
If you are visiting the stunning Grotte de Niaux, this is only 15 mins or so away ....
South of Niaux, head to the village of Vicdessos. Don't cross the bridge into the village itself - head straight on and up. (The village is lovely, by the way, with a town square complete with bandstand and bunting, and a couple of picnic benches along the riverside.) This road has been used as part of the Pyrenean stages of the Tour de France .... hence steep in places with switchback turns! The dolmen is sightposted from Vicdessos, and there's enough room for a couple of small cars to park at the bottom of the track that leads to it.
There are some uneven steps and potentially a bit of a scramble to get to the top - but wow. And wow again.
A substantial chunk of glacial granite, in the most stunning spot, with views over snowcapped mountains in France, Andorra and Spain.