Butts and Gutts hands down and I think I own every one of her leg DVD's and for me this one is the "DOMS for days" DVD!! It's fun and got tons of options.
I love butts and guts and legs and glutes too, but STS Plyo wins hardest for sure for me too. Although, these new spinning w/o look super difficult too. Probably depends which muscles you want to focus on, and what is fun for you.
Plyo legs is the only Cathe lower body workout I haven't done (yet!). They're all pretty tough, but I think Pure Strength Legs is the hardest of the ones I've done.
Cathe is reputable for producing enjoyably toughest workout on the market.
The hardest/Goodies for me are triset weight lifting program (LIS and STS), Floor workouts (Butt and Gutts), Plyos and the notoriously ouchy barre programs Turbo barre .
To Jennifer---You are going to fall in love with STS plyo, I promiss you .
Pure strength----That was my first experience of Sit and Stand those do fry cores, quad and glutes. So effective though
The Pure Strength series (if it was considered a series back then) was my introduction to lifting heavier weights and I've never looked back! Yes, those sit and stands, oh my...ouch!
I never thought about which is the hardest for me. I guess it depends on my goal. I find the Slow and Heavy leg section with the vest on and heavy weights to be pretty tough-and use it when I'm in a strength building mood. When I'm in endurance mode I love Butts and Guts and Leaner Legs. But you all know they're all good and get the job done!