I need help deciding on which champion to buy

Rek sai is the best jgler atm, gnar is good and zac is well meh but he has new skins/chromas.

I think you should buy Gnar. I have him and i like playing him alot. but i guess its on which playstyle u like Gnar is good for tank and some dmg with the black cleaver. Rek'sai is good for tank and good early ganks (I basing this off what i've heard, I haven't played him yet) Zac is very good for a very "In your Face" Tanky engage and has good synergy with other champs.

Rek'sai has the biggest presence. Run teleport and smite when you jungle with her and you can split push top lane, ult onto bot lane before you get ganked, then teleport back to top lane before they can converge on you bottom lane. Rek'sai also has the greatest potential to carry a game. Decent damage output, great mobility across the map and a respectable amount of CC on her kit. If I had to recommend just one out of the three this is my pick.

Zac is a utility tank. He can't really kill stuff without back up but he brings so much crowd control to the team that his teammates will have a really easy time picking people off after he disrupted them for a bit. His damage output is lackluster and his clear times are slow but he is a really good initiator for team fights.

Gnar is useless. Do not buy. Do not consider buying. Gnar's build path is very strict and predictable. His play style is inconsistent. One minute he is a fleet-of-foot nuisance and the next he is a slow as hell kitable tank. If Gnar builds damage he will die before pulling off his full combo as Mega Gnar. If Gnar builds tanky he has horrible damage output when he isn't mega-gnar and will have horrendous waveclears. Oh and he doesn't have an ultimate if he's not in his Mega Gnar form. He has way too many weaknesses to be a considerable choice for any role.

all Gnar players except Dyrus are cancer