I haven't seen much outside of that brief wave of "extreme" French horrors of 2000s. I didn't really like any of those. In fact, I thought they were pretty terrible. People rave about Inside and Martyrs and (not so much) Frontiers, but I thought they were all garbage. Less than captivating attempts at juxtaposing art and extreme violence. Beyond that, I'll try to think any French horrors I know of.

High Tension - Despite what I just said, this movie is artsy and violent, but I love that dreamlike mood. The context of the whole twist ending nullifies any previously perceived "tension", but I was pretty obsessed about this movie for a minute there.

Like you said, Diabolique was good. A classy old school movie with a touch of the unsettling.

Shiver of the Vampires - The first Jean Rollin movie I saw, but it sets a high standard. A hippy, Hammer-esque, and most importantly, sexy movie. Fortunately, I mostly remember liking the movie, which means a rewatch should yield a fun experience.

Irreversible was good. Mostly in the reverse narrative technique, but still disturbing enough.

I've exhausted the well. I definitely want to see more Jean Rollin movies, I know that much.