Instead of worst selling title, why not try to figure out which was the biggest money-loser?
Messiah for example cost a ton to develop (took years), and I hear it bombed in sales. Daikatana (yeah I can already hear the snickers across the internet) was damn expensive too and probably sold even less. Considering Ion Storm almost got completely wiped out by all the combined losses and the Austin devstudio closed down, it had to be a LOT of money...