Purely statistical (leagueofgraphs)
Left strongest, right strong still but less so.
Incredible synergy when the ADC boosts winrate by around or more than 10%.
Keep in mind that there being few synergies does not mean that the support is bad.
Instead, there being a lot of synergies means that the support's full potential is actually inhibited by a lot of champions or activated by the synergy champions.
Few synergies means that the support is well rounded and less reliant on the ADC pick.
Fully updated for 12.12