Titanium in the 6Al/4V and the 3Al/2V alloy type, which are different in their own ways, the material I really wish to try out and own.

Carbon fibre, not bad a material if Lance Armstrong (hey legstrong, I thought of that nick before, hehe) is riding it. It is said to provide a ride like steel but better. Carbon is one material that has great damping properties, that is why many forks are made of carbon fibre.

Steel is not bad, but heavy. I don't have too good a experience with steel though, it rusts! The one I had didn't exactly provide a smooth ride, maybe because it was rusting. Others have said that it provides a excellent ride, hopefully I can try one of those out, but it won't be my dream bike since it will be slightly heavier. I'm a lightweight rider, can't afford to carry too much mass.

Aluminium, good experience with it. It is a good ride with my new Trek 2200. It damps the vibration better than my previous steel hardtail, maybe because of the carbon fibre fork, not sure.

Anyway, all I want to say is that all bikes are different for different people. A 100kg guy riding on a sub-1kg frame will definitely be complaining, it will be flexing too much. A 40kg guy on a very stiff aluminium frame will also be complaining since he will be the shock-absorber of the bike. It also depends on the manufacturer and the other components on the bike that handles the road vibrations, like the tyre pressure and wheel, the saddle etc.