You can't go wrong with either MNKE's or AW's. The Panasonic CGR18650CH is a good flat top, and I've had good results with the NCR18650PD as well. If you get the AW's, get the 1600mAh variety. They hold voltage better under load than the 2000 mAh.
Having a good charger is just as important as having a good battery. I recommend either a Pila or Xtar charger. The Nitecore Intellicharger is a good economical choice. You will want to periodically check your batts fresh off the charger to make sure they're not overcharging, so a digital multimeter is a must if you want to ensure maximum battery life. You can also check resistance if you're building your own atomizers.
If you've got the scratch for a premium kit, kill two birds with one stone and pick up an Xtar VP1. It's a precise charger and it shows you the charge on your battery as it's working.
edit: Protip: ALWAYS read through every single link Baditude shows you. Read it all. He'll know if you didn't, and bad things might happen to you.