For a long (long) time the best free signing was Gary Mac, who was a bit of a surprise due to how much of an impact he made during the treble season (or as Mourinho would call it, the quintiple).
But the ongoing, top level game changing and dressing-room shaping influence of Milner has probably eclipsed that. And he's still, what, two years younger than Mac was when he joined? I hope Milner stays on another year, though I suspect he's such a competitor that he'll want to move on to a more busy role before too long.