User blog comment:RoznosicielCiast/Let's talk about unique champion synergies/@comment-4636093-20150225005835

Soraka plus another heavy healer such as Kayle, Nami or Taric turns the entire team unkillable, or at the very least unsiegable. If the other heavy healer focusses exclusivly on healing Soraka then she can pump out more heals for the team than any enemy team can deal with.

There is also the dreaded Urgot Taric bot lane from season 2, which ironically has two of the most underused or underpowered champions in it but is still holds enough ultimate synergy to be one of the strongest bot lanes. Clicklock stun + Hard to land but devistating skillshot. ADC who relies on being tanky + Support who makes him tanky. Etc.