The silk versions usually look the most realistic to start, but are extremely hard to clean so they end up getting ugly and algae covered pretty quickly. As far as plastic versions go, the Penn-Plex "Jungle Pods" that you find at Petsmart or on Amazon seem to make nice, fairly natural looking displays. Marineland and Blue Ribbon also tend to have a good variety of natural looking options.
Generally the aquascaping seems to be the most important factor on how natural you can make your artificial plants look. If you place the artificial ones in similar patterns to an aquascape you'd see in a planted tank (in clumps, artistically, etc) then the tank can look quite nice and won't have the typical "fake" feel to it.