Hello!
So in ranked I'm maining jg, and I have a fairly decent champion pool
This consists of: Rek'sai Zac Lee Sin Rengar Kha'zix Gragas
However, I have the issue of not being sure when to pick who, and that's what I'm asking, when do I pick who?
Thanks!
Most of your champions fit into any teamcomp- Rek, Zac, Lee, Grag. Zac excels at initiating teamfights so if you just want heavy initiation go for him.
I wouldn't take lee or reksai as LP into a team that has no real initiation i'd go with grag/zac
Take rengar/khazix when the enemy team has a plethora of squishies for you to choose from, they both SUCK against tanky comps. Being against Double ADC comps are best for these guys.
I would add an AP jungler to your pool
and of course 
Learn amumu and rammus
