Being completely new to this I have no opinion on what is the best material to use to construct a machine as I simply haven't done enough research to be able to form a reasonable opinion yet but if you were to ask me which of the 3 main materials I would prefer to use then I guess the answer would have to be wood followed by aluminium & steel would come last.
This is based purely on the equipment I have personally & my own experience working with each of them. Sure everyone is different but personally I have the most experience working with steel but have next to no equipment I would be happy using to construct a machine from steel. When it comes to using wood I have the equipment I need & am confident using plus enough knowledge to be able to go ahead with a build. Aluminium falls somewhere in the middle as far as knowledge goes but not 100% sure about equipment & the price is high compared with the other 2 but I still feel I could do a better job with aluminium that steel.