Mostly 15 and 12 Slots, though I have the odd 8 or 10 Slot in some places still. My main also has a singular 18 slot (a Safe Box for my 3 Alternate Armorsets). Then of course there's starter 20 Slot on everyone.
Keep my bottom two bag slots Invisible Bags for storing alternate weapons and armorsets, aswell as the odd harvesting tool or consumable. I like Oiled Bags and Crafting Bags, though I plan to ditch my Equipment Bags when I can afford too seems they don't really seem to help with organisation much and just get confusing when I swap from a 1H+OH weapon for a 2H weapon and the game chucks one of my personal weapons that I want to keep in there along with the junk. For this reason an Exotic Bag from FotM would probably be handy simply to catch my own equipment when I swap out, but they are pretty expensive, so.
Anyway, in general 15 Slot bags I feel are the way to go though; Seem to be the best balance between capacity and pricing. 20 Slots quite simply aren't worth it at all when you can get five 18 Slots for the same price. Honestly I could see them halfing the cost of 20 Slot bags in a future update, especially since they decided to unfairly throw free ones out to a small select few of the playerbase that managed to make it to the end of the event weekend. Wasn't really fair on the rest of us who couldn't make it / get the chest for whatever reason.
Edited by KotCR, 23 November 2012 - 08:35 PM.