Brian was probably my favorite due to technical skill and achievement in the field of guitar, however Freddie's charisma, stage performance, voice, and songwriting is what really got Queen to their apex of fame.
Hmm, is this a serious question? I mean, c'mon, Queen is a band, this is not 'The Freddie Mercury Band', with Freddie on vocals/piano, Brian on guitar, Roger on drums and John on bassguitar. No, this is Queen, a band with four equal people, they were all great! So, simply enough, I don't choose a individual. If you are really a Queen fan, than you have to love them all!
None. Honestly, I find all 4 members rather bland on their own. None of their solo stuff was particularly interesting. I found Brian's solo stuff unlistenable. Roger's solo/band stuff was laughable...almost a parody of itself. Freddie's stuff was OK, but certainly not Queen.
Ultimately, I found the 4 members of Queen rather uninteresting musicians EXCEPT when they were together. As Queen, there's hardly a vocalist that can compare to Freedie, Brian is certainly on the top of the guitar-god list, and Roger and John were an amazing rhythym section. This is one of those band where the whole was infinitely more than the sums of the parts.
All IMHO, of course.