Up until 4.0.1, it was JC/Engi.
However, with the recent change to Engineering (nerf haste to hands, but allowed the regular spell power enchant), is there anything more 'powerful' min/max-wise?
Or will JC/Engi still trump all?
It really shouldn't matter too much. All professions have som e degree of uselessness to it, all professions offer roughly teh same bonus. The question is how good it is.
Tailoring in WoTLK was good due to a slighly OP back enchant. The downside was that making money off tailoring was a PITA. In Cataclysm, the back enchants are no longer OP, just on par with everything else.
Jewelcrafting is a strong profession since you can get almost any stat, and it's a good moneymaker.
Enchanting is a good moneymaker, but the self-bonuses leave something to be desired.
Engineering has some fun toys, which often outshine any bonuses you get from profession perks.
Inscription used to be a major moneymaker, and the major perk was that you didn't have to farm sons of hodir rep.
Herbalism is utterly useless as a priest. You can heal yourself well enough already. Decent moneymaker though!
Skinning is not amazing as a priest, considering that it's a poor moneymaker and crit isn't exactly a stellar stat for a priest.
Mining... great moneymaker, but the extra HP bonus isn't particularly necessary.
But ultimately, it don't really matter. Pick what you enjoy.