Leon Spinks had 1 good win. He struggled in most of his other "decent" wins... got his ass kicked most of the time when he fought somebody decent.
Briggs has the weakest claim to the title because even though Foreman was considered the lineal champ, he had been stripped of the WBA and IBF belts and Briggs' win over him was questionable. In his next fight he was stopped in 5 by Lewis but he did rock Lewis early.
Perhaps Briggs in regards to being claim as Champ, but I think Spinks takes that title on skill level.
I would say Leon Spinks. A lineal champion has to have one top win in order to get his hands on the title, but with all the others there is at least one other good win you can point to.
I would argue that the following champions are clearly not the weakest lineal champion: Marvin Hart, Jess Willard, Primo Carnera, Jimmy Braddock, Ingemar Johansen, Shannon Briggs, Hasim Rahman. While they are among the weaker lineal champions, they all have multiple really good world class wins.
Leon is certainly a top candidate. Briggs too I personally would choose accidental champ Hasim Rahman. He got totally destroyed in his first defense.
Leon had two good heavyweight wins, Ali I and Mercado, and a decent rallying knockout of Evangelista to come back from the Coetzee disaster. One of the glossy Victory Sports Series boxing periodicals had a photograph of him posing on their cover during his brief reign with the caption, "Is there anyone who CANNOT beat Leon Spinks?" He did not impress in the draw with LeDoux, the close win over 27-0-0 Alfio Righetti which got him to Ali, or the draw afterwards against Eddie Lopez [where a dumb flagrant retaliatory head butt attempt by The Animal cost him the win]. Marvin Hart knocked out HOFer Jack Root, decisioned a young Jack Johnson, halted Peter Maher in two, and competed on even terms with HOFers O'Brien and Choynski. Having 20 round stamina, consistent hustle and respectable resilience and toughness, I see him clearly defeating the similarly sized Leon head to head on most occasions. Carnera's legitimate streak of title wins over Jack Sharkey, Paulino Uzcudun and Tommy Loughran is good enough to eventually get him into Canastota, as the most active heavyweight champion between 1910 and 1938. He came back from Max Baer with a fine road decision over Campolo in Buenos Aires, and back from Louis with a legitimate fourth round win on cuts over Neusel the second man to stop the 43-3-5 Blonde Tiger after Schmeling. Hobbled with a bad ankle after the first knockdown from Max Baer which would require hospitalization, he nonetheless got up to win all rounds in which he was not knocked down. I don't see any version of Leon beating Primo in a legitimate contest.
To be fair he got destroyed by one of the most complete performances and one of the best punches I have ever seen in heavyweight history
Don't forget, however, that he also got destroyed - twice - by Oleg Maskaev. :yep Rahman is the perfect example of a "one-hit wonder." At least Leon Spinks outworked Ali.