I would second Nami, since she's one of the most adaptable supports.
She can disengage well, she can heal, she can engage well, and she can actually deal some solid damage, particularly in the early and mid game.
I'm also M7 with her, and have experimented heavily (with pretty good effect) with a Hail of Blades build that makes her very aggressive early, and able to out-trade and win in some matchups even outside of the support role, without sacrificing too much of her support potential (this actually makes her engage potential stronger, as her E slow and full damage is more consistent when she uses it herself, and with Revitalize/Font of Life, you're still marking your target and allowing your allies to heal.)
She's just a super good all-rounder that you can use in many ways. Whereas with Pyke or Thresh: you make the play, or you're pretty much worthless.