You can't beat synthetics. The linen, canvas Micarta variants are probably best. Not because they are soft or easy to grind / file etc. - they are not. BUT, they come very consistent in quality, dead FLAT and of even size and thickness. This takes a lot of guesswork out of the process of getting a good fit, alignment etc. I always suggest the beginner use canvas first - you only need a 240grit finish and it looks awesome - nice, tactical and grip ! If you ever want to test the knife - there are no worries about the handle breaking etc. G10 and CF are hard on tools.
The best woods, I think have to be either stabilised for well, stability and longevity as well as ease of finish and working. The rosewood and african blackwood family are superb handle materials but many people get allergic to their dusts. A very good alternative is the ironwoods, the acacia spp - gidgee, mulga and koa, just as examples - all dense, durable with some water-resistance due to resin and oil content and will take a fantastic finish. Just 2000grit OOOOsteel wool finish and carnauba wax makes for a very serviceable handle that looks good.
Importantly, as mentioned, wear respiratory protection. Jason.