/sch is my personal preference, mainly because of its nuking capabilities and versatility.
As others above me also say, the 'Arts' stances add a significant amount to your Magic skills making it easier to land nukes and enfeebles as well as making cures more potent.
Aside from that, Sublimation is indeed a very nice MP recovery tool if there's noone in the party that can/will cast Refresh on you.
This sub provides both offensive qualities as well as support options, depending on your party and enemy.
Not counting some of your very useful passive traits like conserve MP.
You can play Summoner like Rydia in FFIV!
/rdm is mainly useful for its MP recovery options, apart from the occasional Cure (Which will be less potent due to the notably lower skill, the same goes for nukes and enfeebles) and Fast Cast this sub doesn't bring much to the table in ways of functionality.
/whm, ah, the staple of the level 75 era, these days mostly useful for transportation and when fighting mobs that may petrify your party members.
That said, Summoners are hardly employed as main healers anymore and even if we are, /sch benefits the role much better even if /whm gets a higher tier cure and access to the Curaga range.