Don't know what the absolute best is, but for me its tire sled drags. I made a car tire sled, put a piece of ply in the bottom and bolted a 20kg weight to it. I do 100m drags taking large strides and moving as fast as I can. I do this for 30min sessions and try to match or beat how many drags i got the last time. I do them on my off days. Its killer cardio and fat burn, but most benefit comes from increased metabolism.
It is only looking at a sample size of N=10. They used young, normal weight, untrained, sedentary men. They put 10 subjects on an "Endurance Training (ET)" program (treadmill, 30 minutes per day, 3X per week), 10 subjects on "Resistance Training (RT)" ("max strength", 3 sets, 10 reps, major movements, whole body, 3X per week, increasing weights and decreasing reps), and a control group (CG), that sat on there butts.
They found that the ET group increased VO2 max by 17% and RT group increased by 12% (both values are statistically "significant" increases, and not significantly "different" from each other -- meaning that their sample size is not large enough for them to say that 17% is statistically larger than 12%).
Their conclusion was that ..."whole-body resistance training not only benefits strength but also enhances cardiovascular function." -- meaning that there was significant improvement in cardiovascular fitness alone from only resistance training.
Somebody needs to look at combined RT and ET.
BTW, I'm not saying you should not do cardiovascular (I do), but short of burning calories -- this study suggests it may not do that much for cardiovascular fitness.
I do a combo early morning fasted cardio walking hr no more than 120-130 bpm 5-6 x week & second cardio High intensity workout 2-3 x per week sprints 1 workout- various distances, workout 2 & 3 series of timed calithenics 15 sec on 10 sec off of jump rope, squat thrusts, mt climbers, jumping jacks x 5 sets & rowing 15 on 10 sec off 5 sets of 3 fast as possible and finish with interval stairmaster 20 sec on 30 off incresing up to level 19.