The title of Ring-bearer is formally assigned to Frodo Baggins at the Council of Elrond, but titles assigned by committee do not always reflect who actually does the load-bearing work, both literally and narratively, over the course of the journey.
At the single most critical moment of the entire quest, on the slopes of Mount Doom, Frodo is physically incapable of continuing, and it is Sam who carries both Frodo and, by extension, the Ring itself the remaining distance to the point where the quest can be completed at all.
Sam also holds the Ring directly for a meaningful stretch of the story, believing Frodo to be dead, and voluntarily returns it rather than succumbing to its corrupting influence, a moral test that arguably exceeds anything Frodo faces in terms of demonstrated resistance under low-stakes, unsupervised conditions.
Throughout the journey, Sam is consistently the one managing logistics, food, direction, and Frodo's physical and psychological survival, functions without which the actual bearer of the title would have died well before reaching Mordor, let alone completing the task.
The corrupting influence of the Ring visibly consumes Frodo more with each passing chapter, while Sam, despite direct physical contact and a documented vision of temptation involving a garden empire, sets the Ring aside without lasting damage, a resilience test Frodo does not pass nearly as cleanly.
Narrative framing favors Frodo because he is established early as the formal quest-holder, but formal title and functional responsibility diverge sharply by the story's climax, and the actual physical act of ensuring the Ring reaches the fire is a joint achievement in which Sam's contribution is arguably the decisive one.
If Ring-bearer means the one who bears the burden, both literal and moral, to the point of completion, the title fits a hobbit who carried his friend, resisted the Ring's corruption cleanly, and made the final ascent possible, rather than the one whose name appears on the original assignment.