single lotus, double lotus, butterfly stretch, getting/crossing leg(s) behind the head, touching the toes (standing and sitting) and getting comfortable (includes breathing well) with granby rolling and inverting and people stacking you when you go for arm bars and other stuff from the guard (this means get comfortable lying on your back with your legs folded..
https://www.youtube.com/watch?v=OwmaB5hO_-w the position at 20:50.. if you can hang out in that position fairly comfortable try to straighten your knees, touch the ground with your toes and then try to dropp your butt as low as possible - as close to the ground as you can, while breating and keeping your toes on the ground) also use the "straight kicks" as a dynamic stretching exercise - warm up for other stuff you do that targets your back and the back of your legs..(1:26 in the video)
those are the most important stretches for jiu-jitsu.
also watch this short video..and do this triangle-stretch the guy shows too..(to increase put the leg in front of the knee instead of behind the knee..)
https://www.youtube.com/watch?v=V6xWQCTSe0I