Hello John Phillips,
I am not sure there is a “best” for anything. Studies are always being done and the results are always being tweaked. The options are endless when it comes to training programs.
That is why cross training, like the others mention, is a good way to go.
Mix up the interval training days with steady state training days.
The best type of training just may be whatever the client is willing to do and maintain, for their heart health.
NAPS 2 B Fit