Inhibitor + Leech for teamfight disruption, sustain, and death-dealing
Get these first if the enemy team is too hard to Goo Ball into
Colossus can be bought first for maximum Goo Ball chaos, then coupled with Diviner's Kevlar for maximum resistance and some cooldown reduction
If you're resource poor, Hunter's Aura helps your team secure kills on people you rush down
If you have Inhibitor + Leech and the game somehow isn't over, spiritual overflow will kill whatever organic matter remains in the recesses of the drain.