I strongly prefer the movies that I don't like being riffed to the good ones.
I'd rather watch Troll 2 and Missile to the Moon than Lord of the Rings or Casino Royalle riffed hands down.
I have to say that I'm liking the current wave of RT less than usual. I did really enjoy the Sixth Sense and Spider-Man 3 and Jurassic Park before that. The ones in between have been either OK, or I just didn't like them at all.
BUT I understand you can't please everyone, no matter what the guys release, some one's gonna be unhappy with the choice. I keep buying them, because I know that one in every three or four is going to be gold and appeal to me.
I also really appreciate the Rifftrax Presents line, particularly the work of Matthew, and hope we get some more of him (and others) soon!
Anyway, personally, I think Memento is the oddest choice for riffing so far, though I imagine every scene change is basically a set up... Hopefully I will be surprised!