Apparently, I’m the only one who likes this better than Oldboy.
IMO Oldboy has worse plotting. It isn’t as poetic, beautiful or appealing. It’s overuse of stylization and gore didn’t seem to be of any use to the story beside adding shock value and thrills.

But Lady Vengeance is in a totally different department, violence, when shown, is used to add to the story, not for it’s own sake. The cinematography, pacing and editing are superior in several levels.

So is the characters and plot. From what I remember, Oldboy had a one dimensional main character, differing from Lady, which had a very ambiguous protagonist.

It deals with more than only revenge, it deals with the duality of man, redemption, motherhood, virtue and many other themes.