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If it comes down to brutality and suspense, High Tension. Aja was masterful in combining the ultra violence with the high pitched sounds. However, the twist ruined it for me. I thought it would have been better if they'd stuck with the killer angle.
Martyrs, was an interesting flick. I really dug the premise, and it had some decent violence, but there was a lot of time dedicated to the relationship between the girls, which made the movie lag a bit for me.
In the end, they're dead even for me, but as far as French horror goes, Inside and Frontier(s) above these two, with Baby Blood in first all the way.