This has been a fairly common question that we have seen on these boards now and then and I wanted to do my best to answer which professions give the most to the mage if they are going to be raiding:
In Alphabetical Order and ignoring money saving/making properties -- we are trying to min/max here:
Gathering:
Mining - Small HP boost
Herbalism - 3min HoT
Skinning - +25 Crit Rating
Crafting:
Alchemy - Mixology (4hr Flasks and +37 SP using Frostwyrm)
Blacksmithing - Extra Gem Slot on your Bracers and Gloves (Belt is usable by everyone). +38 SP with Rare Gems
Engineering - Glove Enchant that gives a haste buff when used
Enchanting - Ring Enchants (+19x2 SP)
Inscription - Unique shoulder enchants = +36 SP
Jewelcrafting - 3x Dragon's eye gems (Prismatic). = +59 SP and -24 stamina (or -16 spirit) compared to using rare gems
Leatherworking - Bracer Encants = +37 SP
Tailoring - Unique back enchant (Lightweave Embroidery) = +/~2dps
Overall, the best benefits seem to be enchanting and jewelcrafting. Blacksmithing is decent to be able to add the two additional gem slots.