Cheap mats, quick sales and big profits!
Almost all bags are profitable to make, but Netherweave Bags have been exceptionally profitable for a long while now. They also require only 315 skill in tailoring.
To put it simple, you need one stack of Netherweave cloth to create a Netherweave bag. Then just turn the cloth into bolts. Oh, and you also need a Rune Thread from a vendor.
The said cloth is extremely cheap on almost every realm, starting from 2g a stack and rarely going over 15g. The bags, however, sell for 25-50g most of the time. I've sold many for even 100g each if I've been the only seller.
Everyone needs bags
There's so many new alts/mains every day so you will usually find a buyer for your bags withing a few hours, depending on your realm population. Netherweave bags seem to sell the fastest. They're cheap when compared to others and offer more than enough slots for any leveling toon.
And some people need bolts!
Alternatively, you can sell the Netherweave Bolts instead of making bags. Usually a bolt costs about 1.5g to make, yet they sell for 2.5-12g each! To make the most gold, best approach would be to keep the Auction House full of both bags and bolts.