The Cleveland Cavaliers have the best record in the NBA, but Norm Nixon tells TMZ Sports he's not entirely sold they're a real threat to win the championship.

We caught up with the former "Showtime" Lakers star out at LAX this week ... and he made it clear, he's got some serious questions about Cleveland's roster.

He told us he thinks the Cavs are good -- they do, after all, own the most wins in the Eastern Conference -- but he wondered out loud how they'll handle the bright lights of April, May and June.

"Until they win -- until they can prove they can win on the big stage," he said, "that's the hesitation."

Nixon appeared to be far more comfortable placing his proverbial bets on the Oklahoma City Thunder and Boston Celtics.

As for his old Los Angeles team, Nixon -- who played in the City of Angels from 1973 to 1983 and won two NBA championships -- was pretty bullish on LeBron James and Co ... though he admitted the squad has to stay healthy if it wants to push for an NBA Finals appearance.

Playoffs start in just a couple weeks ... stay tuned!