Cranksets are not going to offer as much of a noticeable performance upgrade as you would get if you spent your money on one of the best mountain bike saddles or best mountain bike forks. However, there are some important mechanical gains from upgrading to a set of the best MTB cranks, especially if you have a basic set of cranks.
The best MTB cranks could save a little weight but will likely be the key component to upgrading your bike to other more modern standards. If you have a square taper bottom bracket, a new set of cranks will let you switch to a stiffer and more reliable external bottom bracket (more on that shortly) and allow you to ditch a double chainset in favor of a more secure and simple 1X drivetrain.