With a modern mountain bikes, look for ones with:

- Air Suspension Fork
- Thru Axles
- Hydraulic Disk Brakes
- Deore level or above
- Tapered Steerer

Geometry also matters, but I don't know enough to tell the difference so I usually look at models where I can find comments about good geometry.

I'd say a good beginner hard tail starts around $1200 MSRP. The models that have stood out to me are the Trek Roscoe, Salsa Timberjack GX1 and the Santa Cruz Chameleon. I also like cromoly, but the options are more limited in that price range (Marin Pine Mountain being an option).

Mountain bikes are tricky as they have progressed a lot in the last few years. Technically, I can take my 90's mountain bikes on single track, but I won't get into that.