Personally, I prefer Grasp of the undying.
I'm currently using Zac to climb to gold, and then possibly higher once I get there.
I average at around 65% ish team fight contribution, and win 58% of my matches with him.
I find the best time to gank to benefit you, and your team in the long-run is to go in for a gank at level 3 for any of the lanes that are either:
Over-extended by the enemy.
Both your team-mate, and their opponent are sitting around 20-40% health.
W can really be helpful when ganking three, or more enemies at once. I typically have everyone retreat from a turret when they are at low health, wait behind a wall, then when the enemies push up onto turret, I sling-shot over, knock up, w, collect as much of my blobs before positioning myself behind them to ult them further into our turret so my team can engage them with turret assist. imo, I believe that is thee best way to shut-down a team-fight as Zac. Plus if you go into passive, you are right under turret with your team. the safest way to force a one-sided team-fight.