Marvin Bagley III should DEFINITELY be the #1 overall pick in the NBA Draft on Thursday ... so says his father, Marvin Bagley II.
"He should [go #1]," Bagley told TMZ Sports out in NYC ... "He's the best player in the draft!"
Marvin's dad says the whole family is excited for the 19-year-old superstar -- saying they're all trying to take it one day at a time.
We also asked if he was concerned about fake friends trying to leech off Marvin after he gets rich -- and Mr. Bagley told us there's a plan for that.
"He's just gotta keep the circle tight!"
It's good advice -- but as we all know, easier said than done.