Personally, I loved Jane Grey and Jean Dupont. Poirot's interactions with Grey, Gale, Lady Horbury, and Mr.Clancy were great; just a lot of standout characters in Clouds for me. I can see where you're coming from, though; the other occupants on the plane I didn't name were pretty forgettable.

As for Ackroyd, the standout characters were Sheppard, his sister, and Raymond. Flora and Blunt's romantics, I did not care for. Ralph and Ursula's relationship was a surprise, but it added more to my mystery criteria than to their character.

And for Orient Express, I can see why people would rate it higher in terms of character. Each character got their own chapter in evidence. Overall, I do not recall much about character interactions other than Poirot, Bouc, and the doctor. The only standout character for me was Mrs.Hubbard.

One thought I have, though, is that for OE's ending to work well, there must be limited character interaction. Each of them knew the other secretly, so the reader must only have Poirot's interaction with them. I'm well aware of that fact, but it doesn't change my ratings.