Well we've certainly felt the need to buy replacements every season, which has not been the case for other positions which kind of suggests the managers have been less than enthusiastic about them.I'd look at our goals conceded columns for those seasons and i bet there aren't many better.
We attack, with a lot of players. Yet we don't concede many. I'd say that the defence was doing a pretty good job all in all (Fullbacks last season were a liability but thats sorted now).
A lot of the players that would come under the 'best' are protected by the formation, think of how defensive Juve are... It wasn't a given that someone like Bonucci wouldn't have got exposed for us.