I don't think I've yet experienced the ultimate handle material. I particularly don't like slick metal handles, though what Buck has done with their titanium handled 560, giving it circular grip cuts and fingergrooves works better than most metal handles.
I'm less and less a fan of soft rubber and various synthetics like Kraton, since they don't seem to stand up to solvents particularly well, and tend to chip out in chunks over time. (No more Kraton CS's or Kershaws for me! ;(
Various woods, are perhaps the best looking choice for a using knife, but they aren't as durable as I would like. Laminated woods solve many of the durability and stability problems, but I can't help but think of them as plywood or something only found on lowend Frost Cutlery stuff, even though I know that's not accurate.
I'm a big fan of Micarta and G10, though some of the best handle designs I've got are found on lowly thermoplastic. I'd like to see more Micarta and G10 handles formed into some of the better shapes found on many plastic handled kitchen knives. Actually, what I think it all comes down to is shape, anyway. What I want are knives that aren't slick or slippery and have good enough fore and aft protection to guard against slippage, coupled with a palm filling ergonomic design.
I know the knife makers don't like that much shaping of stuffs like Micarta and G10, and I know that would add to the cost, but I think that's got to be the next direction of handle improvement. Show me a line of folders and hunters with Micarta, G10 or better handles modeled on the Victorinox/Forschner Fibrox handled kitchen line, and I just about guarantee that they'll be a pleasure to use for hours and hours.
BTW-- This is neither here nor there, but if I had to list 3 handle materials that I simply detest, and will not buy, they'd be slick stainless, mother of pearl and oosic. Ick!
mps