Ask a hundred vapers this question and you'll get a hundred answers. The real answer is "whatever works for you"; everyone is different.
That said, my best advice depends on how much you smoke now. If it's just a few cigs a day, like half a pack, a cig-a-like would probably be enough. If you're a pack-a-day or more smoker, Id skip them and go for something bigger. I smoked 1.5 packs a day & started on the Joye 510... it worked, but the battery life (or lack thereof) was really frustrating. My first upgrade was a few months later to the eGo, and that served me well for quite some time. If I didn't like to tinker and fiddle with stuff, I could probably have stayed there forever.
If you smoke a lot, you're gonna want something eGo sized or better. Other than that, it's personal preference & you may have to try more than one particular model before you hit the "sweet spot". Don't forget that there are also different types of cartos, tanks, juices, etc., which can really change the experience in and of themselves, so DON'T GIVE UP if the first one you try just isn't "cutting it". Keep trying, and you'll find your zone.