For me it was Ali, because I always feel that for people to put their heart and soul into something that will ultimately ruin you, means so much. Repeated head trauma can only spell a bad end for your career - you can have all the millions, fame, recognition as "the greatest", but in the end you're just a walking vegetable. Biggest respect, and hardest loss for him and all who loved him.
Bowie was huge, but felt distant. I never "got" his music, but respected what he contributed to the genres he played to and inspired. For me, Prince was much more significant because I recognized his talent easier, maybe it was just more on my level. The tributes to Prince and his style of soloing were quite beautiful.
Yelchin though, holy shit. So young, so much going for him, and so freakish an accident, just made me realize how fragile life is, and you can never let your guard down.