The best would probably be one of the top of the line VV
mods since it'll give you many more options than a 5v. But if you're only wanting 5v there's quite a few options there too. If you want inexpensive try one of those listed here
Custom Mods. Of course a 5v PT with a 2 amp wall adapter is even cheaper from them. If you want something a little classier try
GLV 2 from Great Lakes Vapor. There's a 5v option in one of the dropdown menus.