Taking the essentials, puncture stuff, lights, main clothing etc out of the equation... the most useful cannot do without item I have bought is my bikehand maintenance stand. 10 seconds to put up and hours of fighting with the bike on the ground saved. Apart from basic maintenance and odd fettling, I have stripped down, rebuilt and setup all three of my bikes, most recently my roadie using the Bikehand. Just this afternoon, in 45 minutes I have degreased the cassette, gears, chainrings n chain, washed and re-lubed then adjusted the gears. Without the stand I would have added an hour, lots of swearing and at least one Elastoplast!