Not ranked in order of importance
1) Product Selection (more choice the better)
2) Product Price (No explanation needed)
3) Product Quality (This ties in to some extent to the price of course. If flower is £100 an oz its not likely to be as good as £200 an oz flower for example)
4) Delivery Service (Do they post the day you order if its early enough? Do they mark it as sent immediately and then post the next day?Do they give tracking if asked?)
5) Delivery Stealth (no weed smells)
6) Comms between vendor and yourself. This can be very important if you have never used a particular vendor before...or if there are any delays with your order arriving. Do they reply promptly? Some vendors will just swap out your order for something similar if they have run out, others will message you and ask what replacement you would like etc.
Virtually every day someone posts a topic about X vendor not replying to their "where is my order its not arrived yet" message
As for who the best is,
https://www.youtube.com/watch?v=dQw4w9WgXcQ