As a heavy smoker, you are probably going to need long battery life, so you might want to avoid the cigarette-sized minis.
The Riva is a good choice for starting out, but you might also need more power in the long run.
That's when you might want to consider getting into the larger models.
If you really, really want the best though, the current answer is probably either the ProVari, the Darwin, or the Reo Grand.
But I wouldn't shell out that kind of money until you've learned more about what it is you want and need from an electronic cigarette.
What concerns me more, for you, is making sure you get high enough nicotine strength.
I think you should start no lower than 24mg strength, and you may even need to go higher for a short time.
So I suggest getting a bunch of samples sizes in 24mg strength, and maybe a few in 36mg strength just in case.