I've mostly been using V2, which I like for a lot of reasons, especially their prices and customer service. They have a pretty good discount program for customers--once you've spent a certain amount you get discounts on everything. They also have a monthly discount code and a lot of sales.
The other place where I buy a lot of e-liquid is VaporFi. For the most part, I have three favorite flavors, and I carry three batteries most of the time, and sort of switch around. It helps keep me from being bored with the taste of only one.
Both of my sons (in their 20s) smoke, and I've tried to get them vaping instead. Gave them each a $100 gift certificate, but one had NO interest whatsoever, and the other one was "meh" about it and apparently still prefers cigarettes.
One of the other MAJOR advantages to vaping, for me, is traveling, which I have to do for work. It's so great to be able to vape in my nonsmoking room and in rental cars. Never a peep of complaint from anyone. I also love not reeking of tobacco and not burning my furniture or clothing.