League of Legends (LoL) Question: Is Zac still good?

Zac is probably the weakest of the three. My unbiased answer would be Kha'Zix, but I think Nocturne is super underestimated.

Out of the junglers you mentioned, I'd say Kha'Zix is the strongest since he deals ridiculous damage during his ganks and can snowball pretty hard, especially if he gets resets on his leap in teamfights.

Personally, I feel Zac is still pretty decent both in the jungle as well as in top lane (I prefer playing him top). He clears the jungle pretty fast, his ganks are good both pre-6 and post-6 (mainly because of Elastic Slingshot) and he can be a real nuisance in teamfights. Having said that, however, there are still stronger junglers out there, such as Elise and Vi.

Zac is still good, but he is a lot stronger in the top lane than he is in the jungle.

If you're thinking of a jungler to buy right now, then you should go for Kha'Zix. Kha'Zix is a very fun champion to play due to his massive damage, and resets on Leap once evolved. He is a monster when it comes to dueling most of the other junglers because you can usually get isolated damage on to them. Additionally, his jungle clearing ramps up over time since you're building a lot of damage. Lastly, Kha'Zix snowballs like no other jungler allowing for him to be one of the few junglers that can carry games practically by himself.

Here is a jungle Kha'Zix commentary if you are interested in learning how to play him: http://www.youtube.com/watch?v=2tEsgNdk_0s