It really depends on how you define "best". And on personal preference.
Both G-10 and Micarta are made by almost the exact same process (in all honesty, you can think of G-10 as being a very specific kind of Micarta, made with glass fiber, instead of the linen, canvas, and paper used in traditional "micarta" (Carbon Fiber actually falls in the same subset, but using carbon fiber as it's fabric base)).
Each one gives you a different appearance and a different feel. The appearance is a combination of how the handle is finished, the colors used in the base material, and the color of the epoxy resin used. The different feel comes from both the base material and the finish, with the finish honestly being more important, IMO. I've felt rough and grippy G-10, and I've felt Micarta that was polished to an almost glass-like smoothness. I've also felt the reverse for both.
Personally I've always felt that micarta seemed a little 'warmer' in the hand, although that could be my imagination. My general preference leans towards G-10 for folders, and micarta for fixed blades (well, unless you factor in natural wood, because I'm a serious sucker for natural wood handles). For a comfortable, grippy, hard wearing fixed blade handle, on a medium to large knife, I'd probably lean towards canvas micarta. For anything else, it's going to depends on how it looks, how it feels, and what kind of mood I'm in that day. ;-)
[In other words, if you take care of it, and avoid the really "fancy" handles (like mother of pearl), pretty much any handle material you find will probably work well and last for many, many, years. It is true, however, that synthetic materials will generally last longer and wear better than most natural materials. Micarta, G-10, whatever, they're amazingly durable.
