Which 8 week program is the most challenging?

Hi everyone, I’ve recently become a member after discovering the great workout HIIT Like a Girl And 10 min Ab exercises on YouTube!

I wonder if anyone can give me some advice about which 8 week plan to purchase? I love doing long and Intense workouts and was just wondering if anyone knows which 8 week programme would be the most challenging? (Which one has the longest HIIT sessions and will leave you most tired in a good way). I usually feel like 21 or 16 mins HIIT (not including warm up or cool down) is slightly on the short side.

Would FB BURN, Fatloss or Fit be better to purchase?

Thanks so much!

Lisanne