This was a very simple choice, really. The only game that I felt would stand out to be the TurboGrafx-16 Game of the Year in 1990 would be Bonk's Adventure. It really is one of the best platformers out there (in terms of graphical style, I perfer Bonk's Adventure over Bonk's Revenge, and perhaps Bonk 3: Bonk's Big Adventure, too). It had also brought along a mascot for the TurboGrafx-16, giving it an extra earning for this honor.
The other good choices would be Dragon's Curse and Devil's Crush, but in all terms, I feel, Bonk's Adventure wins either way.