I've don't think I've played anything really bad either. I will say that the only one I couldn't get into was Avernum but, to be fair, I haven't tried the later versions of it, just the original, and that was extraordinarily dated in all ways by the time I played it. I think they're (or is it he? Seems like a one-man show for the most part) into the third version of it now. I've played Avadon I and II by the same maker and enjoyed it.
Some stick out; as mentioned prior, Gothic I,II, and III are wonderful games and most seemed not to like the series since the original team was bought out but that (and this was said above too) doesn't mean that it isn't a good game - only that it differed significantly in many ways from the earlier ones.
Another that I feel stands out is (after the user patches like the "True patch Gold" or the latest "Unofficial patch basic" have been applied) Vampire the Masquerade: Bloodlines.
--- and, of course, we all know of "Original Sin".
