I don't really mind the overload of flashbacks, though at first in Noir it was pretty redundant to the point of annoyance. It definitely added to the mystery and development of its story.
I'm not sure you can watch just one and see them all. Like the three girls-with-guns series, they're pretty different despite its similarities, at least in style. Noir is more psychologically focused, Madlax is plot focused, and El Cazador is... um. character-developmentally focused (lawl, how many plotholes where there again?). At least that's how I think of it. .hack//SIGN is a lot more psychologically and character-developmentally based than plot, even though the plot was pretty important to the series. Spider Riders is more of a kid's show too (I'm sure they botched it when they dubbed it for Fox Kids, though).
(I tried watching Avenger. Not that great, I think.)