Very few Avengers from comic book history stand any kind of chance against Thanos in one-on-one combat - especially not when the Mad Titan is wielding the Infinity Gauntlet - but Sentry is certainly the closest thing the superhero team have ever had to a Thanos-level threat. There are indeed a number of powerful heroes who could yet be brought in to help in the final fight against the all-powerful villain - Nova, Quasar, Ms. Marvel etc - but none of them come close to the power levels of a prime Sentry. Robert Reynolds, as he is also known, is essentially a Superman analogue whose powers are even more potent than the man of steel - but he also possesses a number of other powers. For example, his Molecule Man-countering matter transmutation powers could be a decent way to counter the reality warping abilities that Thanos could possess with the Infinity Gauntlet and his mental abilities could be a good way of countering any telepathy that Thanos could bring to the table. Basically, if the Avengers need a big gun brought in to combat the big, purple, maniacal extra-terrestrial threat of Thanos, they don't come much bigger than the Sentry.
