Poll: Best support in for corki?

For protecting Corki, Janna.

However, best overall synergy (out of options here) perhaps Leona or Thresh for their high CC.

Impossible to answer - it depends too much on who you are going up against.

**Corki is pretty straightforward as an ADC - he's very 'all-rounder' style, so he pairs well with pretty much every support in the game. **

Why is not on this list?

I would argue Zyra, but maybe that's because she's so oppressive and I'm a terrible adc in general so I just view her as strong :^)