With Genesect's sole weakness being fire, the douse (water) drive would seem to be the best option, as it hits all fire types supereffectively (bar Reshiram), which also resist Genesect's STABs. This is really only remotely viable in-game, and pretty much worthless in the online competitive environment, as hidden power water and a boosting item will outdamage it, while also boosting Genesect's other moves. On the DS however, this may be used, but Genesect's fantastic movepool including U-turn, bug buzz, iron head, explosion the elemental beams and even shift gear and extreme speed through event, you will struggle to find a reason to put this weak move in. However if you really want to use Technoblast, douse drive is the best choice in my opinion.
Hope this helps.