I think the reason Mass Effect 2 is so high on the list, is that most people here don't think of it as much of an RPG - and I agree with that.
But it's kinda misleading, because it's established as an RPG (as much as anything) - and it doesn't really matter if people personally agree with the genre definition. In fact, I don't think it makes sense to rate "Best/Worst RPG" based on how much any individual - or group of individuals - thinks it belongs in the genre. I just go by how it's being marketed and labelled by the developers - and they should be in charge of what to call their own game. Not that I even know what Bioware calls it, but it's certainly enough of an RPG to me, to qualify for the genre - but then, I don't need much for that to be true.
Personally, I think genres are only really useful as a guide to the kind of experience you might expect, and nothing more. What really matters is whether it's a good game or not. But that's me.
As a game, I think Mass Effect 2 is pretty good - and I think it's kinda unfair to have it be 2nd worst RPG.
But, maybe people really think it's that bad as a game - overall - I just doubt it.