Son of Charles Xavier, David Haller/Legion is an Omega Level mutant, and quite possibly the most dangerous one too. He has the powers of telepathy, telekinesis along with Reality warping at will and time travel. But this is not what makes him dangerous, his split personality with each personality having different powers and some even un-controllable by David himself. If Thanos and David get down to get dirty, Legion could and probably would warp him out of existence.
Sentry/void is basically Marvel’s answer to Superman.(not a rip-off) Robert Reynolds when becomes Sentry, he possesses Superhuman strength, speed, stamina etc. along with the ability to manipulate light and become intangible and some other abilities. Sentry is basically unkillable. He was even cremated by Thor by throwing sentry in the sun when he begged to kill him, But Sentry came back even more angrier and stronger.