Which chain guide suits your bike?
Once you have decided to use a chain guide, you will run into a large range from different producers. First, you need to be clear as to what type of drive train or frame the chain guide is to be attached. If present it is best to affix the chain guide to the ISCG/5 mounting plate. In doing this you need to establish whether the frame has an ISCG mounting plate (triple threaded tab in the pedal area) If you have a mounting plate you need to additionally work out whether it is a ISCG (old) or a ISCG 5 (new). There is unfortunately no clear rule as to whether the frame has an ISCG mounting plate or not.
With the ISCG 5 the mounting plates are slightly further apart, they are mostly used with Press Fit brackets. If you don't have a ISCG plate, then some guides have the option of affixing the guide to the seat tube or the the front derailleur mount. If nothing is possible, then some chain guides can be, in an emergency, wedged under the bottom bracket (BB).
Next it is important to decide, whether you need an adjustable guide or not. So whether you use 1-speed, 2-speed or 3-speed. Depending on the use you can choose a guide with Bash/Taco (chain ring guard). For all Mountain/Enduro bikes that use a Narrow-wide chain ring, either no or only a light upper guide is needed. For Downhill use we would advise using a complete guide, both top and bottom with Bash/Taco.