Ring Resistance
SPOILERS contained within:
Who do you think had the highest resistance to the Ring?
Candidates:
Bilbo - possessed the Ring for decades, remained mostly intact, although he had started to display some signs of decay/corruption/addiction. Worth noting: he was far from Mordor, Sauron was not yet returned to full power, and he used the Ring infrequently and for generally selfless/benign reasons.
Frodo - carried the Ring for years, "owned" it for longer. Brought the Ring (mostly) on his own all the way to Mt. Doom at the height of Sauron's return. He did wind up corrupted, however briefly, at the very end, though.
Sam - was nearby to the Ring for quite some time, never corrupted or displayed signs of corruption. Carried/wore the Ring for a short period of time, close to/within Mordor.
Saruman - never touched the Ring, but desired it and its power. Still, he fought against Sauron and the forces of evil for centuries before being corrupted by a lust for power, and he was using a Palantir.
We could also speculate on Gandalf, other members of the Fellowship, and Galadriel as people who are near or directly offered the Ring. Boromir, Isildur, and Gollum are clearly not front-runners, as they were relatively quickly and totally corrupted by the Ring.
For my money, it's Frodo. He didn't own the Ring as long as his uncle, and he sank further into its sway, but he was also burdened far more than anybody else who claimed it - bringing it at the height of Sauron's power right to his doorstep. I've gotta figure the Morgul wound wasn't helping any.
As pure as Sam is, he wasn't subjected to the Ring's direct will for nearly as long as his beloved master.