I played the demo for MGS4 and didn't get on with it at all but put it down to a bad demo given that I generally really enjoy stealth games. I had it sitting in my unplayed pile for a while until I did a brutal clear-out of stuff I didn't think I'd ever get round to and that was one of the victims of the cull.
MGS4 was the weakest of the lot, despite surprise guinea pig ambushes. I'd say each has been progressively worse, although I played the gc version of the first.
Many would say that 3 was the best, but I missed the radar.