Which monster do you like the most in Heroes 5 (level 6)?

Definitely better. But you have to keep in mind that there's no unit at level 7 in HoMM V that's worse than the Archdevil...:(

I completely agree, but with the right strategy, it's invaluable!
For example, if you spend the whole game increasing the speed of your units in every way possible, with artifacts and skills... and gating! You can resurrect powerful monsters from the dead with the Archdevil and move around the entire map, which wouldn't be possible without those bonuses! With this setup, you can easily corner a Nightmare and finish it off with the Archdevil. Overall, it's awesome!