MOUNTAIN BIKES

There a lot of factors to consider when buying a MTB but it really boils down to what type of riding you like to do,... and your budget.

For a great all-round bike a hardtail mountain bike might be a option. No rear shock also means they are often lighter than a full suspension bike at a similar price point. You also get better part specs over a full-suspension bike at the same price.
On rough terrain and for more comfort on long cross-country rides you might want to look at a full-suspension bike. While hardtails may usually be the lighter option, the advantages of an active rear end make it sometimes worth a weight penalty on more challenging trails.

There's also the option between carbon and aluminium but this weight difference isn't as crucial as it might be on a road bike.