First of all, I wanna thank you, Qvist, again, for making these lists. It is always a pleasure to see a new list posted.
On the ruins ranking itself, I'm not quite convinced on the ranks #2, #3 and #4. I have a feeling that people overestimate the probability of having that magical great engine that just lacks +buy, which you then obtain via your ruined market. +1 card seems a bit underrated here to me. While this is not an accurate comparison, consider a random card that would either get +1 card or +1 buy. Certainly, in almost all cases, you'd rather have +1 card than +1 buy. This is also reflected in real cards (compare e.g. Smithy - Hunting Grounds - Margrave). This kind of comparisons aside, I believe that just those many occurrences of "meh turn in a ruins-heavy game - 1 free action left and no action card except ruins in hand" in which you'd rather have Ruined Library (so that you can actually buy a single decent card, you don't even worry about buying two cards) swings the balance in favor of Ruined Library, but I stand to be corrected here.