I have already decided who ____ the contest.
I thought of ' is going to' because he/she has already decided.
both parts of the sentence should be in the present perfect tense.
I agree with " Kati Svaby", that he/ she has already decided
"going to"' mean similar as "I plan to do something", so If somebody else decide somehing about the other person its not the other person plan.
Anyway why we don't get the correct answer after the vote?
I agree with scipapicsa. However, what you have applied is common sense. I haven't taken the time to find out under grammar rules.