Atama no Taisou is missing from the list, it would be up there right after Rainbow Dash.
But from the list you gave:
1 - Rainbow Dash (this map is just too much)
2 - Pluto (harder to FC than Airman even being shorter)
3 - Airman (It's just hard, similar patterns to Rainbow Dash, but higher AR and lower BPM makes it easier)
4 - Big Black (Too easy to miss something)
5 - Sono Chi no Kioku (short, but hard)
6 - Senketsu no Chikai (only the stars after the break are actually hard)
7 - gangsta (easier than all above)
But from the list you gave:
1 - Rainbow Dash (this map is just too much)
2 - Pluto (harder to FC than Airman even being shorter)
3 - Airman (It's just hard, similar patterns to Rainbow Dash, but higher AR and lower BPM makes it easier)
4 - Big Black (Too easy to miss something)
5 - Sono Chi no Kioku (short, but hard)
6 - Senketsu no Chikai (only the stars after the break are actually hard)
7 - gangsta (easier than all above)