II. There are other sub-par entries in the series, but honestly II is barely even playable, especially the original NES version. It's a shame too, because it has a much better story than I or III, but its levelling and battle mechanics are so ridiculously ill-conceived that they just kill the whole thing.
XIII would be a close runner-up, as it's just far too monotonous. Sorry, but if I have to wait a length of time the equivalent of an entire playthrough of any of the first three Mass Effect games until it "gets good", it just isn't worth the effort to get into it.
(I suspect I wouldn't have thought very highly of XV had I played it on release, but I bought it a year later with all the DLC, and actually thought it was pretty decent. If nothing else, I had more fun in the first hour of playing XV than in all the time I slogged through XIII)







