[otaku1313]

In order of difficulty (AC, meaning on pop'n 7-9 and Best Hits you have to set the difficulty to 3 instead of default):

Classic 7 (43 in pop'n 7 CS, also in pop'n 8 CS)
Moushima (13 CS)
SCREEN (Best Hits/pop'n 10 CS)
Classic 9 (pop'n 9 CS/pop'n 10 CS)
Classic 8? (pop'n 8 CS/pop'n 9 CS)
Cowboy? (pop'n 11 CS, slightly easier timing in Iroha CS)
Toy Contemporary? (Party AC)

Toy Contemporary is the only 43 from an AC mix
Cowboy and Moushima are not in any AC mix
Classic 8's EX is not in any AC mix
Classic 8 is the hardest 43 to pass consistently BY FAR, and on an AC mix, Japanese players might say it would be harder than Cowboy and Toy Contemporary. The life difficulty on pop'n 8 and 9 are incorrect for current AC mixes, and the closest you can get is turning it up by 1. Even then, the lifebars aren't as harsh with life recovery as AC mixes.
Cowboy is probably easier to pass on stock pop'n music machines than Toy Contemporary, thanks to very hard-to-press switches.
If it was on a home version or if you were playing on a machine with modified switches (as many larger arcades in Japan do), some might call Toy Contemporary easier than Cowboy.

I've personally passed all of the 43s except Cowboy and Toy Contemporary, and there are probably 200-300 people better than me in Japan, based on size of the top Taisen level and how many times I won/got 2nd/got last. That means probably 2000-3000 people could have passed a single 43 or more, considering how easy Moushima (or Classic 7 if you count it) is.

EDIT: Didn't see the last part of the first post. The difficulties from 41 to 43 aren't spread out more because it's quite possible 10 different people could pass 10 different first 41s and 42s. There are a few exceptions like ProgBaro and Niente for 42s and Lamento for 41s, but that's because they're supposed to be a step before the next difficulty. There were no 42s (well, rated 42, since Oi Punk was a 41) in pop'n when the first 43 appeared, which is why some 42s are harder than most 43s.

Here's something for you to think about: Is BALLS-OUT ROCK EX easy for you to pass? My first 41 was Classic 7 EX, and I didn't pass anything above 38 with a lot of chords until pop'n 10 came out. Later on, Kenka Drum was a really easy 42 for me to pass but Enzetsu and Fever Robo gave me huge trouble. Up until Carnival (which I hadn't played when I played FEVER in the arcade), there were only a few 42s, all of which were very difficult for me, so I was left trying to pass all the 41s. During that time, my scaling ability stagnated and my chord ability went way up. It all depends on how you progress through the game and what you're good at at the time.