One usually kn I ws who the top dog is in a given division. I pay little attention to "lineal" this or that.
Two parts: 1) The guy who is retrospectively accepted as champion by the overwhelming majority of current boxing historians. 2) Provided that the modern consensus overlaps with public opinion in the champion's era reasonably well. So Wlad was a champion, even before he got all the belts, arguably. Holmes was a champion even though he dropped some belts. Sometime between 67 and 71, Frazier became champ, even though he wasn't lineal. Johnson was champion when he beat Burns, even though Jeffries retired undefeated. By contrast, Firpo could never become champion, even if ultra-revisionist nutcases took over IBRO, because nobody at the time accepted his claim.
an Honest & Earned Career leading to said Title and always fighting the Contenders you are expected too. no fighter can fight everybody, especially in the last 50 years with 25 - 40 fight careers, but IF you're not meeting the True TOP Contenders to get there, you're NO Champion!
I can't think of a case when having unified all the belts wasn't enough, but I don't think you always have to. Holmes was champion with just one belt, Holy was champion after beating Bowe even though the WBC wasn't included and Lewis continued being champion after losing some belts. I wouldn't say lineal is always enough, though. Wouldn't call Cotto or Briggs the champion for example and wouldn't call Fury the champion now.
Artur Grigorian - WBO lightweight champion in the 1990s and 6th greatest lightweight of all time (Duran is 8th). That's as real as it gets.
The one Champion (in his weight class) of the one world (planet earth) who is the man to beat. Just like in a movie: Rocky Balboa who has to face Apollo Creed.
