Look for a Niner Air Nine aluminum. Stay away from anything with a Lefty lol, unless you like constant maintenance and problems

  
Any hard tail 29'er will do fine. Get one with 100m travel fork, and decent components. Alum or carbon, either are fine. Only other thing to add is a dropper post and you are all set for just about anything. I like the Scott Scale, but really any 29'er in the right size with decent components would feel about the same. The big differences are in the full sus bikes but the full sus bikes are big money.

One other thing you might consider is a hard tail "plus" bike. Would be harder to find used as they are pretty new, but cool bikes.

FAT CAAD is a lot of fun

any alloy hardtrail with ~XT or X0/X01 would be solid. Frame differences among alloy hardtails account for very little in terms of tactile benefits... Fork/brake choice along with component group (ideally 1x, for simplicity) and swapping tires depending on terrain will make most frame differences irrelevant.

Have a look at Jamis, they make some very nice "value" bikes, that ride great.

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Take a look at the new Spec. Chisel Expert. They know what they are doing with frames, the 100mm Reba is pretty bomb-proof in my experience, GX is hardy 1x11 system (and 1x is the way to go). Only thing I would change is to swap the SRAM brakes for a set of XT's ($100 and done).

Another vote for a Niner or the new Specialized Chisel.

Be forewarned, going off-road is addictive!

I had an 18 year old mountain bike and this year bought a Scott Scale 940 - it is so much better in so many ways than I had ever expected.

It is an XT 2X equipped aluminum 29er with a Fox float 100mm ft fork and 29er tubeless tires. I have ridden it about 40 times including doing an Xterra.

Scott has more selection in hard tails than anyone, check out the scale range. The bikes I would consider would be 940, 945, 950.

It is so much easier to ride and more forgiving than what I had before.

I paid $1800.