#1 Throat hit, strong or don't bother
#2 Vapor, I like watching it swirl around, I always did the same thing when I smoked.
#3 Flavor, I like good flavor, but it took years for our gear to get good enough that I could really taste my juice, so I just got used to it being the least important part of the equation.
Honestly, I'd say vapor and flavor are more of a combined #2 these days, but my priority is definitely a nice solid, punchy throat hit or I just don't get any real satisfaction. It's the main reason I've never tried to wean myself down to zero nicotine.