Best Support?

Pick Alistart if he's open. As Alistar you can really widen your teams lead, but can't do much if you're behind, so be careful.
If you rather wanna play it safe and have a decent engage AND disengage go for Thresh.
And if you only want to disengage go for Janna/Nami. Those two are made for staying behind your team and supporting them in fights, or interrupt enemys when they engage your team.