Most crank manufacturers use their own bottom bracket design so if you are upgrading to a different brand, it usually means you need to upgrade your bottom bracket as well. There are a few different standards used by frame manufacturers so this will also dictate which bottom bracket you can use and thus, which is the best MTB crank to upgrade to.
The best way to find out what you need is to identify what you currently have fitted to your bike. Alternatively, you can check with your frame manufacturer's tech specs or ask your local bike shop for advice. If you are unsure as to what to look for when checking that a bottom bracket is compatible be sure to read our guide on the best MTB bottom brackets.