I'm probably one of the few who can be entertained by a ridiculously evil complete monster if it's approached with some care....and putting a decent backstory behind their actions and mixing a bit of grey in there just sweetens the deal. Though I admit, it is getting harder to make people 'buy' this sort of villain anymore. If they're too typical, they fall flat easily.
I'm going to use Diablo and it's demons as an example for a moment...okay, granted it's a game series with next to NO character development for the villains, but in Diablo 1 & 2, the boss demons were plenty intimidating because they were just this sort of silent, ominous presence inside of this dark and foreboding world that was thick with unpleasant atmosphere, so when you finally fought them, it was a satisfying rush........until Diablo 3. In Diablo 3, none of the baddies ever shut up or said anything aside from your typical villain speeches...so they came off as silly, corny, boring cardboard cutout villains.
Another example is Shere Khan from the Jungle Book. Perhaps not the greatest example either since it's an old Disney cartoon (the version I'm referring to) and they're known for ridiculously over the top villains with little motivation, but there was so much build up to this guy that when he finally appeared, and proved to be plenty threatening, he didn't disappoint. XP Maybe you could argue that he's a tiger and is just kind of doing what tigers do, but he enjoyed killing for entertainment, he enjoyed tormenting and horrifying his victims, so it kind of pushes him into complete monster territory in my opinion. Yet, he was still a cool character.
The more complex villains are still the most interesting type of villain, they're the type we're drawn to the most, but I think the 'big bad monsters' can still be at the very at least an intimidating obstacle if there is a bit of subtlety put into it.
.....plus, I just grew up with a lot of insane big bads, so I can't help but have a soft spot for them. XD