It's worth noting that I'm no shmup pro - I had a fairly hard time with Lords of Thunder for example. I mean, I HAVE finished it, which probably means it's on the easier end of shmup difficulty, but I didn't breeze it and it took me an awful lot of attempts to finally take it out.
If I'm saying one is fairly easy, odds are that it's not going to be considered too tough by any genre vets
Of my list, Cotton 100% is the easiest. It's also a lot of fun, so give it a go.