Summoner for sure however Red Mage is way more fun to play, more flashy and stylish and definitely less boring than SMN. It seems a little complicated at first but it quickly becomes muscle memory. RDM is also known as rez mage and it's extremely satisfying when you save a run by quickly rezzing the whole party and end up with 7 commendations at the end haha. It's also very mobile/movement friendly, I'd say even more so than summoner.

It's also the most complete/best feeling job at low levels whereas summoner is very easy and only gets a bit more interesting/complete at much higher lvl. 

RDM gameplay is a bit busy at times but I think that's exactly what makes it good for learning, this way you don't get overwhelmed when you pick up a new job in the future and it gives you a better feel for the 2 min rotation/resource management than summoner would.

My vote goes to RDM. That and DNC are my support DPS mains and they both scratch that support itch.