What was Ernie Banks? A great shortstop, of course. That's what the Hall of Fame says. Indeed, Banks did play in 1,125 games as a shortstop. But he also played in 1,259 games as a first baseman. Even including his brief action at third base and in the outfield, Banks spent more than half of his career at first base.
Rod Carew, same thing. According to the Hall of Fame, he's a second baseman, but he actually spent more than half of his career at first base. Harmon Killebrew is listed as a first baseman, but spent less than 40 percent of his career at first base. Killebrew is a different case, though; while he spent much of his career in left field or at third base, he did play first base more than anything else. So if you have to assign him a position, that's the one. But of course that reasoning does not apply to Banks and Carew. Based purely on playing time, both should be considered first basemen.