As a general statement I'm not a fan of tank supports in their current state. Some of them can work well enough to be viable, but I just don't see much value in giving your team a frontliner who will be behind in both items and levels compared to what could be coming from Jungle/Top (supports get less income and share exp with ADC, leaving them 2-3 levels behind solo laners in the late game).
Having said that, if you are gonna play tank supports I heavily recommend Alistar and Leona because they can temporarily inflate their defensive stats which negates some of the problems i mentioned above. Leo is fine, just make sure not to blind pick her and you'll be okay (she's easily countered by Morg).
Whats "best" is really too dependent on the situation to answer, but Morg is very safe into just about anything and if you just want something you can blind pick all the time no matter what I'd start there. Just make sure that you get a Zhonyas pretty quickly. If you cant survive long enough in team fights to get your ult off her value goes way down.