I usually give it to the people who I see contributing in the graphics section, that includes the shop section of course and people who can present me with a decent portfolio of their work. Everyone who has the rank at least PMd me and showed me a portfolio with their work, I can only recall me giving the rank to someone 1 or 2 times without them literally asking me for the it and showing me a portfolio.
I don't want to be too strict on who I give the rank because there is already a fairly low amount of interest in creating graphics in this community and making the rank too hard to obtain is not going to motivate people either.
We used to have a designer I, II and III rank with each their different permission sets that worked with a point system based on contribution, winning SOTW/SOTM and so on but it was scrapped because it was simply not worth maintaining due to the low amount of interest.
A public poll is also not the way to go for this I think, it's just going to be a popularity contest and people like @Scotty (no offence) will end up with the rank. What I could do next time is forward the designer application to the other designers and ask for their input before deciding.