LaVar Ball is now taking aim at the OTHER big college hoops star projected to go #1 in the NBA Draft ... saying Markelle Fultz ain't worthy of the top spot -- because he's not a winner.

Draft experts have been debating whether it'll be LaVar's kid Lonzo Ball or University of Washington star Markelle Fultz who'll get the top honor at the NBA Draft in June.

But LaVar tells TMZ Sports it's a no-brainer -- Lonzo wins. Markelle doesn't.

LaVar's point ... Lonzo was a key reason UCLA chalked up a 29-4 record this year. Markelle led Washington to a 9-22 season.