My take would be that i DGAF if other people on the internet/US share my taste in t-shirts. My sole concern in that area would be geared towards wanting a free one. Personally, I wouldn't buy one I wouldn't be happy to pay the full price for, and of course my brain sees it as very unlikely that this would be the least popular. Also, it seems a poor gamble to go for one I like less on the hope that I might get another free. So if i picked the least popular, my reaction would just be 'bonus, free tee!' (i wouldn't have any feeling of 'special unique little flower', either).

As for the language specifically, firstly, 'least popular' is functional. It tells me exactly what you are talking about. If i read 'most unique' i would have no idea how you were measuring that, and would probably feel uneasy about it-- like you were using the phrase as cover to pick whatever you wanted to. (Which is funny, since 'least popular' is already the best financial option for yous!). Secondly, it doesn't seem too negative to me (compare 'least awesome' vs 'worst'). That is for the landing page (and if any commiseration email), anyway. As far as congratulatory email, I would probably agree to frame it as 'most unique'.

Hope that helps!.. I know how difficult it can be to get basic perspective when you are too involved with something.