Very, very difficult question... but I think I'll go with Mayhem. I was introduced to the black metal by them, and I like how their vocal styles vary from album to album.
One of the first black metal bands I ever started listening to... What do you people think? Which one of all the vocalists that have ever performed in Mayhem have made the biggest impression to you? Personally I like each and every one of them in a different way, so it's pretty much impossible to decide.