Unbreakable gets my vote too. It's a tricky idea that could have ended up an embarassing disaster, but MNS managed to pull it off. It had some very powerful scenes.
SS is saved by the ending twist - it came as a nice surprise, after what had honestly become a pretty boring film in the last half hour or so.
Signs has some good, jumpy moments but shows the first signs that MNS's schtick was wearing a little thin. The Village was the first thoroughly bad film, with terrible, expository dialogue that has remained on his films since.