Impact Wise & Cultural Phenoma wise, Baby will always be his biggest hit, because he simply won't ever have another song that has a cultural takeover like that.
Chartwise, and performance wise "What Do You Mean?" could easily be his biggest hit. It's the fastest growing track in Spotify history, it'll probably be his biggest hit on radio, and it's his first #1 hit in US, UK, Australia, New Zealand, Sweden, Norway, and some more countries (+ another #1 in Canada). + it's a comeback, after people swore he wasn't gonna get a hit half as big ever again.