Well, it would be hard to pick a "worst" developer because simply by surviving to produce enough games to the point where we've heard of them, you can't call the product objectively bad. Companies that make absolutly awful games tend to go under fairly quickly.krazykidd said:
If I had to pick, I'd probably point to Zynga, because there is nothing you can say about those games that is objectively good, other than their abillity to appeal to and addict casual gamers. They are however a highly successful company despite the quality of what they churn out.
Of course this is coming from a position of general ignorance, since I don't really play porn games or visual novels. The actual "worst" is probably some Japanese visual novel company that is just barely hanging on by a thread, producing low quality Vor porn for a tiny, but dedicated audience....
Lacking the knowlege to actually name companies like that, though they doubtlessly exist, it's hard to give a definitive answer. You might find someone on say "Something Awful" who could answer this though.
