Wow. I'm struggling on how to say this. I've got lots of affiliate programs - not just for pvs and juices, but with everything. (I really like how Amazon's works.)
There is kind of a psychological thing that happens with affiliate links. People see the "non-straight-forward" links and get suspicious that you are only making this offer to make money.
Sure, I want to be able to make a living, and am always looking for a little extra cash, but there are already too many e-cigarette spammers and junk sites just looking to make a buck off of smokers. This makes us look bad. Take a look at youtube review videos for e-cigarettes and you see people posting a video review of brand x; then, in the description, they have a x% off coupon code listed. It just ruins credibility. Watch the first 40 seconds of this guy's video and it will give the idea of how the public can react. He's kind of funny/crazy.
Affiliate programs work for those who work it, but can actually send the wrong message too. Just look at the rules of this forum. There are no affiliate links allowed. I am assuming it is because of the potential for people to be motivated to sell rather than be a part of this community.
So, to give you an answer to your question, I would say that the best affiliate program would be (WHO) the one that your favorite company has. (WHY) Because you actually believe in what they are selling.