It depends on what kind of mountain biking you intend to do. They range from full suspension motocross-lookin' downhill high jumpin' machines & span all the way to skinny-ish tire gravel/road bikes.
For road/street/gravel/singletrack use, I suggest something with hardtail & air/oil suspension forks with a lockout. You get reasonably well singletrack ability, absorbtion of tree roots & such, but don't lose all your pedaling efficiency with unnecessary suspension that'll never get used to it's full potential.
I have a 1997 Trek 6500zx & run down the full suspension riders on the paths in our woods constantly. They can't pedal as well when their bike is absorbing every pedal stroke...of course they kill me on the rough & can do jumps I'd never consider.
The field is HUGE and endlessly specialized with a bewildering variety of options. My suggestion of a hardtail & air/oil suspension forks with a lockout will yield a relativly inexpensive, generally capable all-rounder. You can buy another specialized bike when you decide which direction you want to go, discover what your local terrain offers & how/where you want to ride.
Sorry, I can't tell you to buy a specific make or model.