ME Flex is very stiff, and a little fragile, but the spike detail is the best looking. By far the best looking flextrack available for HO.
Atlas Code 100 is very nice to work with, quite flexible, tough, but the detail (Spikes, tieplates) is about O-Scale in size. Probably best for hidden trackage. I haven't used their Code 83 stuff.
Peco I haven't used, but their Code 100 and COde 75 is UK outline, and the details are wrong for US trackage. Their Code 83 looks to be excellent (I've seen some examples fo the turnouts, very impressive).
Walthers/Shinohara is in between Atlas and ME for detail, robustness and flexibility. The Code 83 is Walthers branded, the Code 70 is Shinohara, but all of it is Shinohara made.