IV, II, V, III, VI. So much cooler, and screw anyone else who tells me differently! In this way I can dilute the old audio/video quality with the shitty new dialogue and plot holes - actually to be honest now I just watch my edit, then V then III then VI. I have a feeling that when parts II and III of my edit are done, I'll only ever watch it that way. Of course this is all just personal preference and not at all the "correct" viewing order.
If you were going for Chronological, I would HIGHLY recommend playing TFU 1&2 or watching the cutscenes at least between episodes III and IV.