The decision was a total no-brainer -- Cunningham was Big 12 Player of the Year and an All-American with the Cowboys ... averaging 20 points, 8 rebounds and 3 assists per game in his lone college season.

He was an absolute beast at the high school level as well ... winning national player of the year honors at Montverde Academy in Florida.

With all that success at the HS and college level, it was no surprise the guy was going #1.

Of course, draft night is also all about the #fashion -- and TMZ Sports got footage of the guys as they headed over to Barclays Center earlier Thursday night ... with everyone all decked out in some of the best draft day suits we've ever seen!!!

Florida State forward Scottie Barnes is going the LeBron/Zion route -- rocking an all-white look. Congo's Jonathan Kuminga sticks out with some awesome orange threads.

There are also some eye-popping looks ... but ya gotta give credit to the guys for trying new styles!!

Jalen Green pulled off a bedazzled pinstripe suit and makes it look easy ... while Jalen Suggs also had a similar attention-grabbing ensemble.

There was also a tribute to the late Black Mamba -- LSU's Cam Thomas honored his favorite player of all-time, Kobe Bryant, with a special design inside his blazer.