Out of the collection of drivers we have im ruling a few out instantly.
Keke Rosberg is in no way the worst title winner. His season was lucky with both Ferrari drivers failing to get through the season with accidents and Prost and Piquet always blowing up. He scored points in a Fittipaldi in 1980 then through 82-85 he was argubaly the fastest driver only getting the fastest car for the last few races of 1985 when he was leaving Williams.
Button is not great and not as fast as Keke and did win the title because of the car but he’s been a front runner for almost a decade and not always in a car that deserved podium finishes
Beat Jack Brabham in his own team to win the title in 67 against the Clark and Hill Lotus dream team. Almost retained the title in 1968 with a outdated, unreliable and not particularly fast Mclaren but a suspension failure at the last race cost him a 2nd title. Mclaren would never have the fastest car till 1974 when Hulme had decieded he was retiring and past it and during 69-74 he had several dignified drives out driving the car to wins and podiums
So that leaves 3 in this order
1. Phil Hill
Only once did he beat his team mate in the final points standings ina fair fight and that was 1960, His title win in 1961 was aidded by young team mate Von Trips dying in a crash at teh Italian Gp, Won by Hill. Despite a brilliant sports car record, especially at Le Mans he’d be my worst.
Mike Hawthorn and Jaques Viillenuve, who looked a different driver after winning the title will follow him on the list. Hawthorn had several good years prior to winning the title but thats not enough to escape my list of the 3 worst champions