imo there are 2 legitimate "blueprints" for invoker but in reality this hero is outrageously versatile and you will be switching his level order around every game depending on what you need. for example, if people are barely living from your teamfights, maybe you should pump up exort a little. if you need more health regen to stay in your lane early, perhaps level quas another point. is the other team's lane phase killing you with gankers that chase and slow you, like tide and veno? then get an early ice wall. maybe you want deafening blast early...etc
here is my q/w build
quas/wex
starting items branch tango blades, as sinz said. this build is teamfight heavy and is focused on tornado and emp. It is also very strong against pushing teams. cold snap should be used every fight as well.
you want to level quas, invoke, wex, wex, wex, quas or exort, invoke, wex (exort if you didn't already have it), wex, max exort and get invoke at proper levels
typical item progression is phase boots, magic wand, void stone or staff of wizardry (are you going euls, sheepstick, or force staff?), bongos, then finish your euls/force/sheep. next item depends on what is needed. sheep/euls to have both, or shiva's guard, or ags, or orchid, etc.
