I ran an entire adventure with Aranea as the main villains. They had infiltrated an elven village and proceeded to murder and then replace each inhabitant.
I modified an Yrthak and threw it at my 1e group...lots of fun.
An interesting counter-poll (though I'm not sure how it'd work in a poll format, might have to just be a discussion) would be what are the most common monsters used as opponents in homegrown adventures.