Going by the reviews, 4-5 are the best looking of the bunch (understandably) and they're arguably the best in the series as well, BUT I'm actually leaning toward only buying 1-3, because I don't own them on blu ray, so they're not double dips..
btw, I still don't understand what was/is the big deal of the couch jumping thing on Oprah
it was cringe-worthy, yes, but that's about it