The ultimate cross country race fork. I have this one on my single speed. It has an inertia valve so it’s a ridged fork on the climbs and a plush fork on the downhill.

It's plush on the descents and stiff on the climbs. Fox's "inertia valve" knows when the fork should be locked out and when it should be plush. Makes for a great XC for since you never have to lock-it out for climbs, but more importantly, it makes it impossible to forget to unlock it for the descents. I have an F80X on my Trek 9.9 carbon hardtail and my Trek Top Fuel 110 XC race bike. I'd much rather have the plushness and predictable ride of an F80X than save a bit of weight and ride a noodley SID.