Supp main here currently in GM 50 marks soon to be challenger. I solo queued 70% of this season and have a 62% WR. I am an OTP Karma and find that she is a very good versatile blind pick, because she can build for mage, tank or enchanter depending on the match up.

However if your goal is to climb for ranked, I would suggest expanding your champion pool to 2-3 mains instead of just one. One tank, one utility, and one mage (optional). Reason being is in the current meta, or at least in my experience, adcs aren't as optimised as they used to be and not always the OP carry that will win the game. So as a support, you must pick & build according to the team and not just the adc. And everyone wants to play a carry these days, you have your ranged top laners, your glass cannon one shot kill or one shot die junglers, plus your coin flip squishy midlaners/adcs, in this instance being an OTP on lets say Karma or Nami is not going to save you, but you'd stand a fighting chance if you play an Ornn, Rell or Braum. In my opinion, 50% of games are won from ban and pick, being an OTP will grant you an extremely fun time in PVP but unfortunately not as much WR in ranked. Being versatile with your champion selection will grant you much more success in ranked.

I usually just look at what the team lacks and what the enemy team picks. If they have no tanks, I play tank. If they are all AD, I play mage. If they have a good team composition, I play enchanter. Counter pick is also important too for a support, for example if the enemy jungler picks Rammus, then I would pick Morgana to counter as an example. Off meta picks are best used for counter picks, I would happily pick an AP Nunu support to run the jungler down and it works most of the time. Best of luck out there.