It's best the read the 6 post series in the ECF library. But it breaks down to some catagories:
1) Looks like a real cig, small -- Joye510
2) Real cig--808ds--what I use--Bloog or Volt
3) Bigger, bigger batteries less hassle -- ego, rivas
Remember, you will need spare batteries and deal with charging issues and maybe personal carrying cases so the look and how long the battery lasts are important decisions. The ECF library will help you. Then come back and post how much you smoke, how important the cigarette look is to you and if you mind having batteries you may have to charge every few hours and swap.