GG - great thread!
I'm afraid most of my less-than-ideal flights have been thus made so by the actions of other SLF. Rudeness, ignorance, and lack of consideration for others rate highly, as well as selfishness and cavalier disregard for rules, social norms and common courtesy. This goes for pax on any airline and in any class of travel in my experience.
On that basis, perhaps my ideal flight would be on a bizjet - on my own!