I am a fan of DominATP for my cardio/HIIT or NOS Ether level 1. I am not a big fan of stims when I do HIIT (tends to give me cramps in my sides/chest) and those have beta alanine and citrulline malate in them; both of which are proven to enhance endurance. As for a stim based one, Jack3d is great because a little goes a long way and I have used it before with success but now that I am doing all running it does not always agree with me. Oh, and if you really want to amp it up, OxyElite Pro by USP before sessions is pretty ridiculous. Taking it now and no need for any extra stims after my afternoon dose.