I think The Sisters of Mercy sticks out for me because they are the most non-metal band that I have ever liked to the extent that I like them...
And strangely this seems to be common amongst metalheads. They've been covered by Cradle of Filth, Crematory, In Extremo, and Kreator at least.
And I don't believe any of us can explain WHY we like them.
I bet Stekim would hate them though...