The 'smartest' cardio is simply the one I will do! I am a power athlete by nature, so endurance work is always difficult and less pleasurable for me. So, I enjoy the higher intesity work of sprints, etc.
Howerver, as others have mentioned, like any effort it needs to be rotated. Right now, I am in the gym 3-4 days a week. I will normally due my high intensity cardio work on the same day. Sometimes I take 8 hours between, other times I dont. Depending on what I am working. Sprints followed by hard deadlifts is rough!
Then, on the off days I will rotate in slower cardio (normally long walks) so that i am not tapping into my recovery ability to much. I have tried it the other way where I sprint on the off days, but for me it seems I am always tapping the same energy system in my weights and cardio and not giving any period for rest.
Sometimes, what I find effective is to do multiple moderate sessions through the day, like 15-20 mins of slow-moderate rope skipping in the AM, followed by 15-20 mins of walking/light calisthentics in the PM.