So let's talk about Zac
I for one, don't think he deserves the shit he get's about being weak, but I don't think he's good. Other tanks perform his role but better. He's a sustain based tank, capable in the jungle or the Top-Lane, that sounds a bit like Maokai. His main goal is to initiate with a surprise slingshot and provide wide team AoE disruption and CC, that sounds a bit like Amumu. Amumu is the terror of low ELO, and Maokai is a dominating top-lane presence in the LCS. So why no love for Zac? Who seems to occupy a space in between? The answer is he's a jack of all trank trades and master of none.
His engage and follow up ult, while long range, don't have near the reliabilty and ease of use as Amumu's ult. Especially given the knockback increase which can have unintended consequences very easily. He also doesn't do as much continous damage as Amumu. Unless he can reliable get his blobs (which is hard in teamfights) his dps move from his W is pretty low compared to Amumu's AoE.
His sustain, while potent and frequent, is extremly unreliable and suspect to counter-play (breaking his blobs). For being a toplane sustain tank, his blobs are extremely unreliable. If you're opponent is nearby then the blob will most likely land on their side of the minion line, making it very hard to retrieve them unless you want to try and bully your way through, often losing more HP then you gain. This is in contrast to Maokai, who merely needs to auto-attack after X number of spell casts. The counter-play for his sustain? Don't use your abilities. So you are basically trading virtual HP (the health from his passive that he didn't proc off your spells) for actual HP (if he does actually heal). Both are net wins for Maokai, you're either doing less damage or he's healing more.
Some consideration must be given to the fact that Zac is mana-less. Buffed too much and he becomes Vlad 2.0. A manaless sustain bully menace that you just can't force out of lane and who can effectively harass you. I don't have an easy solution to this. However even beyond the laning phase (where mana matters the most) Zac's late game fantasy is just plain worse then most other tanks. He lands a long range AoE knock up, then bounces a bit breaking up the enemy team then hopes to lay down some AoE dps with the occasional Q and E to keep things interesting. Contrast this to Maokai super tank, protecting himself and his team with a damage reduction aura and point and click CC while being nearly unkillable. Or amumu, who's teamfight ults are legendary, and who's dps actually hurts.
