I have actually enjoyed all 5 games greatly(3 on PS3 and 2 on PSP), but I think I enjoyed 2 the most but not by much over the others. It is a very good series and it functions in a certain realm of gameplay, and I really can't see them ever changing it to much, as it would then no longer be GOW, but who knows?
Resident Evil 4 made a left turn for that series, and that game is one of the best games I have ever played, and my hands down favorite of the RE series. I believe this new game stands to be a "pre" title, so they may make some huge changes as it takes place before the other games, so some differant mechanics can be explained away in that manner. I guess we'll have to wait and see.
Chris