Whichever day has the least # of scheduled meetings.
Chiming in on this topic: Never really thought about my most productive day of the week before, so this might be a difficult one to answer: I guess it's Friday, oddly enough, because I'd like to finish outstanding tasks and get things done before the start of the weekend.
@Connor 's answer made me smile.
Btw, I just love Garfield and his Monday mood.
Mondays tend to be slow to start, but the most productive due to minimal meetings!