From reading the title I’d have said Inners…
Failing that Mabie – the first descent in particular is great, easy to loop round for another runn, too.
IME: Newcastleton – good but short
Mabie: Good bang for buck – not as “monster” as Glentress or Inners, but some great sections, one of my favourite trail centres
Kirroughtree – Goes from easy blue to very techy red with nothing in between
Ae – excellent sections, but the descents are too short – lots of them, just not strung together
Inners – bitch of a long climb, cracking descent – Love it 😉
Glentress – Bits of everything, again great and can be joined to Inners for a monster day
Glentrool – not been as no advertised red or black routes
Drumlanrig (not one of the Stanes, but same area) – good but a bit too much like riding in the South of England to go all the way up North for – Flowy singletrack hillsides (rather than mountainsides) in mud!