Not trying to be macabre or anything, but as far as most brutal, probably the Taliban or Al-Qaeda. . .
The way they pervert the Quran, and the fundamental importance of Islam. . .
As far as "best" training, probably the US, Israel, German special forces, UK. .
As far as "best and most brutal" (which is what I guess is what your question actually entails) prolly North Korea, Russia, or China.