I was honestly surprised at how easy most of them were. I've always had it in my head that they were some of the hardest games out there and had never beaten them without cheats or save states on an emulator before. Each game has its tough as fuck moments, but Three was really the only super hard one for me.
I was surprised how much I liked Four also. I've always had a negative view of that one. But, I thought it was cool that it was the first to really have hidden items/weapons and some branching paths.
I think Six was probably the low point for me. Awesome graphics, but the Robot Masters were pretty lame and just felt like they were going through the motions level design-wise. Some of the levels felt super short as well. One had pretty short levels as well, but that was excusable since it was the first and all.