What's best? That's like asking "what's the best car?" It depends on personal preference, so you'll get 10 different answers. Ego/510 types actually run at 3.2 volts; KR808 models and most basic mods run at a true 3.7 volts. I'm testing a little 3.7v box mod I made last night as a trade (getting a coffee maker from a co-worker for it, lol)... it uses 510 threading, but a 3.7 volt battery, and so far these little boxes I'm building are the best ecigs I've tried.
If you're looking for an 'upgrade', you might search for a mod. Most use 510 threading & run at at least 3.7v. Plus, the batteries are generic lithium-ions, and thus cheap relative to proprietary ones. Caveat: if you get over 5 volts, standard atomizers & such won't work (the built-in protection circuit in the batteries cuts 'em off)... you need specialized HV attys for 6v.