Overall I am going for +healing or +spell power as the most valuable as a holy paladin. Pretty sure the math will back that up but I have absolutely no inclination to crunch the numbers. But Int is nice too as a h pally, because we have a talent that increases % int, int gives crit rating and crit rating is a free heal for h pallies, and mana pool is not nothing.
Spirit is great between pulls (and the horrible experience that is h pally soloing) but in raiding you will usually just be assigned to tank heal and spam a downranked flash of light non-stop and thus becomes less valuable.
So in my humble estimation healing/SP > Int (but not by much) > spirit (while not useless definitely doesn’t compete with SP/Int).