I would go with Haste but I wouldn't class that as a White Magic (and it varies anyway from game to game) so I'll go for... Cure. I know it's common as muck but it is single-handedly the most useful spell. Esuna is up there as well.

Life magic is really far down the list as the most useful yet useless spell. It seems great, but it often has rather high MP cost, and in games with ATB a phoenix down is often quicker and more efficient than using a Life/Raise. In FFXIII, using a Raise costs a full three ATB bars, which in a fast paced game like that can cost you. A phoenix down is instant use. Same with FFXII; a phoenix down is instant and with the right licence grid allocations is better​ than Life.