The sack artists are cool, but they rise and fall from season to season.
The shut-down corners are great at isolating one player.
My vote would have to go toward a play-making safety or a rangy MLB; they cover more than one player, have blitzing, zone, and man responsibilities.
That being said, my vote is for Patrick Willis.
He's been in the league for three years, and has lead the league in tackles for two of them, and came in second place for the other.
Oh yeah, and he's a beast.