One of the pivotal moments in Marvel history has to be when the Fab 4 first encountered Sentry #459 at his lonely outpost in Fantastic Four #64.
This issue by the legendary Stan Lee, Jack Kirby and Joe Sinnott team marked the introduction by name of the Kree.
The "defeat" of the Sentry resulted in the arrival of Ronan the Accuser the very next issue, and eventually triggered the deployment of Captain Mar-Vell to Earth. Captain Marvel himself battled Sentry #459 soon thereafter. Eventually this same Sentry came into conflict with the Avengers a few times, first as part of Ronan the Accuser's tactics in the Kree-Skrull War and later as part of Heavy Metal, a gang of sundry robotic foes. The Sentry even went on to battle the obscure Marvel hero Blackwulf.
What I like about the Sentry is that he is relentlessly loyal to his orders, a force to be reckoned with truly. Here is a cover gallery featuring some of Sentry #459's greatest battles. My feeling is he's still out there waiting for further orders.