I really like Volcano as a one stop company, even though I admittedly end up going elsewhere because I piecemeal everything.
especially if your talking about non mod 510 type units
Quality: They have the best warranty for ego style units that I know of, a year warranty on all batteries
Price: Its competitive, especially when you get into the fact you can get your tanks, your ciscos, and your clearomizers all in one spot, they seem to care about what they carry (even If I personally don't like the tanks enough people do)
Shipping Time: One of the best, they have never come in last when I order from multiple vendors on the same day...and they have a next day option
Selection: Like I said above, they have a good selection of 510 stuff they seem to support their Inferno line with the most popular items at the moment (I do wish they had boge LR cartomizers), I like that they sell the LR Cisco 306 and LR 510's on the site as part of the line.
That being said I often end up going to madvapes because if your shopping for mod supplies and not just ego/510 stuff they have a wider selection of goods and good prices.
other than those two I just look around until I find the cheapest for what I am looking for. Most often my monthly order looks something like this 1) One drip tip from nhaler because I want the fancy green twisted drip tip+ a bottle of juice or some cartomizers to justify shipping 2) some juice from a juice only vendor 3) a battery from some non ecig related site for the mods 4) some super special product x that only vendor y carries and probably a bottle of juice to justify shipping.