I have to go with 4,5,6,1,2,3.
There’s some important plot points in the original trilogy that would be spoiled if you see the new trilogy first. Similarly, there are certain plot points in the new trilogy that depend on certain knowledge from the original to really be appreciated. Finally, because as others have said, since Episode I is just not all that good, if you start with that you may not really be interested in continuing. What makes it at all worth watching is the investment one already has from having seen the other films. So I really just can’t see going 1,2,3,4,5,6. Yeah, it’s chronological, but there’s plenty of examples of stories where the natural progression of the story isn’t chronological.
I’ve also seen suggestions for 4,5,(1),2,3,6, but I don’t really see the point of that. Without spoiling any plot for a Star Wars virgin, obviously 6 is the end of the story, but again, that doesn’t mean it’s the end. I also think there’s plot points in it that will be spoiled if you see the trilogies, though obviously no where near as badly as straight chronological. I guess you do get to have the sort of cliffhanger last longer, but that’s about all the benefit I see out of it.
That all said, I don’t think that for someone who has never seen any of them they’ll think the new ones are as bad because of the complete lack of childhood nostalgia that many of the rest of us have. I do think 1 and 2 are easily the worst of the 6 films, with 1 being largely unnecessary and even having some downright stupid parts that were thankfully largely absent in 2 and 3. I still think they’re enjoyable, even if not particularly good, and they really shouldn’t be completely ignored.
So my on top of that, I’d also probably say not to try to watch them all in one sitting, or even one whole trilogy in a sitting. I’d say go through the release order (or even whatever other order you decide on) and watch roughly one a week. You do need some time to sort or process it and get an idea of the world it’s creating or I think it’ll be a bit difficult to really grasp why it got so popular.
Also, there in the debate of whether 4 or 5 is better, I don’t think it’s close at all. Sure, 4 is a more or less a complete movie on it’s own and 5 is not, but that’s what makes 5 better, that it is really the film that ties the whole saga together. I also think it generally has better writing and directing.