High intensity by far gives you the best gains for the time you put in, but there's only so much you can do without overtraining.

The way to really maximize your potential is to do high intensity 30 mins 3x/wk and then intersperse it with a great deal of low intensity cardio. Like 4x more if you can, per the 80/20 rule.

It works because it's starting to approach the most exercise your body can handle. (Although some athletes definitely fit in more.)

I built this HIIT timer tool www.hiitmeup.fit for exactly this reason as I try to squeeze in a compact exercise during my busy work days. I put this out on show HN a few weeks ago and hope others will find it just as useful as I do.