I will not argue that Battletoads NES is probably the most brutal game out there along with Megaman 1. With Megaman 1 it was SO unforgiving and there were NO saves so you had to sit down playing it all the way through.
Would have to say though my "hardest" game is a toss up between Bayonetta and DMC3 on Dante Must Die mode *original not special edition*. Bayonetta was brutal but I still remember dieing so many times in DMC3 until I finally got the hang of it. Even then it was beyond brutal as you cranked up the difficult to DMD mode. Facing Virgil for the first time on DMD was a blast as the guy used his phantom sword attack in tandem with his other attacks.
Dealing with Bayonetta it was challenging due to the necessity to his those action quick time events as fast as possible. Even dodging which was a core element to the game became tricky but was still fun.
1. Devil May Cry 3 - Hardest setting Dante Must Die Mode (Leaving out the 1 hit = death thing cause that was just stupid imho)
2. Bayonetta
3. Megaman 1
4. Battletoads NES