Rengar, Sejuani, Gragas, or Reksai

I want to add another jungler to my collection so i can have more variety and maybe main any of these. I have tried Rengar, Sejuani, and Reksai but never Gragas. Which do you recommend? I kinda enjoyed rengar when it was on free but im not sure how he does this meta. Reksai is really fun to play and i love tunnel ganking then knockup cuz thats almost always a kill. Sejuani I didnt like atm because she was so tanky and i didnt know her abilities scaled off health.

I currently own : (as of junglers)