magnesium is best ride ive had (litech tubed, not the cast kirks).. smoooooth
tried carbon, aluminium, various steels, currently riding ti.
only aluminium i could stand was the ceramic matrix stuff (m2, m4 etc), all others are too harsh. never ridden one long enough for it to fatigue tho.
not tried scandium which is simular to magnesium cos they are an aluminium alloy also. same for titanuim
basically its about how the crystals form when it solidifies, scandium/magnesuim/titanium make the aluminuim less brittle so you can make the tubes thinner, but they also have an effect on how vibration is carried throu the tubes. "steel" frames are usually steel alloys, eg cromo, mangmo, niti for same reasons.
"alloy" frames are "aluminum with a bit for something in it to make it less crap" just usually not a lot. when they put a lot of something in its called something else, like scandium, or titanium.
carbon is essentially epoxy plastic with the carbon as reinforcement, like fibreglass. again it transmits vibration in a slightly different way, with more damping, better for the 'buzz', but feels dead under you imo.
weight ive found to be a lot more variable than just the material its made of, but as a rule, carbon is lightest, then scandium/magnesium, then aluminium and ti about the same, steel a bit heavier.
overall though gotta be titanuim. no fatigue, no corrosion, plush ride, looks uber.
ping