Absolutely love the original two films, but as for parts 3, 4, and 5 I'd say they are a tossup. I only saw parts 4 and 5 one time back to back and just thought they were damn awful. They weren't funny as the original two films were, from what I remember the last two were just stupid comedy that seems to satisfy the masses these days. Definitely forgettable, and as for the third entry I'd say check it out since you have not done so yet. I should probably re-watch one of these days actually, it is streaming on netflix I believe lol. So my ranking from best to worst would be as follows:
1) ROTLD (1 and 2 tied)
2) ROTLD 3
3) ROTLD (4 and 5 tied)
Let us know how your viewings go and what you think, @SkeletonCrew.