Why only defenders from the last 20 years? Madrid have had success back in the day too.
Real Madrid had essentially four great eras: the di Stéfano era (late 50s and early 60s), the Butragueño era /
La Quinta del Buitre (mid to late 80s), the early Raúl era (late 90s and early 00s) and then Perez's second stint (mid 2010s). In all of them their defenders were never that important, Ramos was definitely the most critical and his partnership with Varane was definitely the best.
In the di Stéfano era for example you have Santamaría as the standout, but that was mostly for being such a consistent feature in winnings teams. It wasn't a great era for defenders really, and it wasn't a feature of Madrid in particular. If you look at their defensive record in their first European Cup win for example, they won 4-3 on aggregate against Partizan in the QF, 5-4 on aggregate against Milan in the SF, then 4-3 in the final against Reims. That was before they brought in Santamaría, and he did tighten them up somewhat, but the feature of that Madrid team was unbelievable attacking power.
In the 80s, Sanchís was the leader and very influential, but in their 5 league titles in a row, they only had the best defence once. It had more to do with Butragueño, Hugo Sánchez and Michel again. It is kind of curious how they never really had a defensive era, or at least not a successful one...club's do have some kind of character that persists.
You'd probably have those three plus Santamaría and Sanchis in the top 5, none of them were as important as Ramos.