I was thinking of doing a little marathon (I've never seen Lady Vengeance yet and have the "fade to black and white" version.)
From what i understand, there is no real continuity between versions (with the exception of the theme of revenge.) But what to watch first, Oldboy came out in the states first, but Sympathy for Mr Vengeance was made first. (i also just got the 3 disc tin from ddd)

So any opinions?

I'd watch in order of production, to see how Chanwook-Park develops as a director.

All 3 films have nothing to do with each other aside fm two things:
1) revenge is the main theme
2) some actors do cameos in all 3 films

I personally would still view them as they came out: SFMV, Oldboy, SFLV.

Keep in mind that eventhough Oldboy was one that seemed to have put Park Chanwook in the mainstream, the level of intensity and nihilism is different amongst the trilogy. I personally found SFMV absolutely engrossing but incredibly draining & dark. Oldboy is probably the most accessible and the most action packed, vengeance here is "in-your-face." SFLV may be the weakest but still packs a wallop, it also shows how a woman scorned cooks up a scheme to get even...manipulative, cunning, patient and undeterred.

oldboy last. it's one of my all time favs. save the best for last.