I'll weigh in on that debate, I also own all three.
Sympathy for Mr. Vengeance is the weakest. As Ikrik said, having a deaf/mute character is kind of hard to get around. His motives don't feel as valid as everyone else's because he's not the one saying them. Because of that, the movie feels slower than it actually is. That, and a couple plot points felt slightly shoehorned in. Other than that, the cinematography is just as good as in the other three; Park just doesn't use as much color correction.
Oldboy is still my favorite. If it weren't for Choi Min-Sik, it probably wouldn't have been my favorite. Storywise, not considering the performances of the actors, Lady Vengeance is a little more poignant. The lead being reunited with her daughter and all the parents getting their own retribution is a little more moving than a man bent on the destruction of his captor. However, the buildup and climax of Oldboy was executed damn near perfectly and the actors did a great job. Plus it was mad stylish.
My complaints with Lady Vengeance are that the little girl's song on the way out to the school kind of yanks you out of the movie for a bit. Some of the performances were a little forced, and it is kind of hard to ignore the similarities between it and Oldboy. Park Chan-Wook sure likes his Vivaldi. If you can watch the last two several months apart so that they're not at the front of your mind it's easier to enjoy them as seperate movies. It basically is Oldboy Lite, but there's nothing wrong with that; but it'll stick out if you're used to watching ultra-stylistic/violent movies for not being as intense.
