Should I get reksai or zac for a tank jungler?

Easily Zac. Rek'Sai has been beat down with the nerf bat too much

Get Zac. Reksai is amazing and her strengths are far better for teams of 5 who work well together with comms. Her vision control is amazing. She's a pretty good CC bot and her global map presence (thanks to ult to tunnels) is good. However, Zac's engages are better and his slingshot is the longest range non ultimate engage in the game. Plus he's a pretty great team fighter with his ult and % max HP damage (W) and can have some great burst for a tank.

Zac, but personally if you wanna be a pain to most adcs try Rammus.