I like firm. Firm. FIIRRMMuh. My favourite "bed" was one of those workout mat (think yoga mat) on a concrete floor.
I got my mattress at Ikea -- they keep changing them, though, I don't think you can get my favourite one any more. It wasn't particularly heavy for a twin, the one I got for the queen bed I use now was a little heavier.
(shudders) @ Torpedo's "whore bed" description -- yeah, I'd hate that too.