Try them all (using Live USB sticks). You only need to use them for a few minutes to see the most significant differences. I personally have been using XUbuntu (ie Ubuntu + XFCE desktop), which is almost identical to Mint-XFCE. A lot of people like the KDE desktop (KUbuntu and Minti-KDE). I personally do not like the mainstream Ubuntu desktop. To some extent it also depends on your computer hardware and your personal preferences. Find one you like, install, and enjoy!