Dragon Ball, One Piece, Saint Seiya — these franchises share comparable escalating physical/energy power logic, making internal "strongest" debates at least coherent.
Series like Death Note define "strength" as strategic and intellectual dominance rather than physical power — not comparable to combat-power rankings.
Hunter x Hunter's Nen system makes power highly situational — a character's ranking shifts depending on matchup and rule exploitation, resisting flat ranking.
There's no objective answer — power systems differ so fundamentally between series (physical combat power vs. intellectual strategy vs. contextual ability systems) that cross-series comparison isn't a coherently answerable question.
Saitama is written specifically as a parody of the power-fantasy genre — his strength is intentionally limitless as a joke about the genre's escalation tropes, which makes him an outlier rather than a comparable data point.