The National Basketball Association is a week away from completing perhaps the most competitive regular season in the history of the post-expansion era. And heaven only knows what's going to happen in the playoffs.

"In just about every year, you had a pretty damned good idea of who the super teams were," said Phoenix general manager Jerry Colangelo. "Now, nobody even wants to guess at a winner. It's gonna' be wide open. Hey, the whole season has been that way. It's been crazy."

Indeed, it has been a year of significant change in the NBA. Could anyone possibly have predicted that Phoenix, one of last year's championship finalists, would now be 15 games under .500 and eliminated from the playoffs.

The world champion Celtics have struggled all season to stay above .500. The Golden State Warriors, with 59 victories and the best record in the league a year ago, are in third place in the NBA West with a chance to win no more than 47 for the season.

There have been other major flip-flops. A year ago, the Lakers, with much of the same personnel but a different coach, lost 42 games. They currently own the best record in the league.

The Houston Rockets, two games under .500 last year, merely have the longest winning streak of the year (nine games), a 47-31 record and the lead in the NBA Central.

The Portland Trailolazers, eight games under .500 a year ago, are now 45-33 and in the playoffs for the first time in the club's history.

The Midwest Division has had the biggest turnaround of all. Detroit, 36-46 in 1975-76, its now 42-35. Kansas City, 31-51 a year ago, is 40-37 and locked in a down-to-the-wire race with Chicago for a playoff spot.

The Bulls? They're only the most improved team in the league. They won 24 games under Dick Motta a year ago. They can win as many as 45 if they streak to the finish.

"There's only one way you can explain what happened," said Philadelphia general manager Pat Williams. "You spell it ABA, and I'm amazed as anyone.

"When they first came in the league, well, I was an NBA purist. I scoffed at that league. I knew they had some great players, but I had no idea about guys like Don Buse (Indiana), Dave Twardzik (Portland) and Claude Terry (N.Y.) Nets).

"I've been wrong. I would say that a year ago that league was equal to the NBA. I wouldn't say it then, but I'll say it now. Look at the Kentucky team that folded, guys like Artis Gilmore, Maurice Lucas and Caldwell Jones. They'd be frightening."

One only need look at the current NBA rosters to understand the impact the merger of the two leagues last summer has had on the professional game.

The Philadelphia 76ers, with former ABA players George McGinnis, Julius Erving and Caldwell Jones leading the charge, will probably be slightly favored to win the NBA title.

Former ABA players acquired in the dispersal draft or by trades played significant roles in the turnarounds at Chicago (Artis Gilmore), Houston (Mosses Malone), Portland (Maurice Lucas, Twardzik); Kansas City (Brian Taylor, Ron Boone, Jim Eakins and Detroit (M. L. Carr, Marvin Barnes, Ralph Simpson).

And, of course, two of the four ABA francises swallowed intact by the NBA have enjoyed consistent success. Denver leads the Midwest Division and San Antonio has been in the middle of the Central Division race all season. The Spurs may have been even better if guard James Silas had been healthy all year.

The numbers also help explain the impact on the old league.

Ten of the 20 top leading scorers, six of the top 10 rebounders and five of the 10 leading shot blockers are ABA alumni.

"When I first started in the ABA, the quality just wasn't very good," said Billy Melchionni, a former player and now assistant to the president of the New York Nets.

"But our young players were playing every day because we had nobody else. A lot of the young guys who went into the NBA sat for a year or two and they stopped developing completely, or they improved very little. Our guys got better and better.

"Last year, I suspected the teams in our league were very tough, in fact, I thought overall we were stronger. Of course we only had six teams. But the Nets last year could have beaten any team in basketball. And St. Louis, our last place team, had guys like Moses, Lucas, Beone and Don Cheyney and they couldn't even finish .500.

Many NBA executives believe their league is going to become even more balanced.

"I doubt if you'll ever see another team win 65 games in a season," said Williams. "And maybe not even 55. The draft and the free agent situation will make it very interesting."

The Lakers for example, will have three No. 1 draft choices in the upcoming draft. "It's hard to believe," said Colangelo of Phoenix, "but right now this is probably the weakest the Lakers will be in the next five years."

Teams will also be able to improve their lot by signing some of the 60-plus free agents now playing out their options.

Leonard Robinson, Tom Henderson, Pete Maravich, Randy Smith, Jamaal Wilkes and Bob Dandridge, among others, all now fall in that category, and all should attract the big money and add to the league's balance.

Some of the teams finishing out of the money this year should also improve next season.

Phoenix lost its top three forwards, Curtiss Perry, Garfield Heard and Keith Erickson, to injuries for half this season. All will be back.The Knicks will be a factor with Bob McAdoo and a healthy Spencer Haywood able to play together and the other have-nots most likely will dabble heavily in the free agent market.

Still, not everyone is entirely pleased with the way the merger has worked out. John Begzos, general manager for San Antonio, says he often feels like an unlove stepchild when dealing with other clubs.

"In the ABA, we had a real cameraderie, I guess because we knew we were always on the brink of disaster," he said. "Some clubs in the NBA just haven't accepted us, teams like Boston, the Knicks, Philly and Detroit still don't consider us equal."I'm not bitter about it because the NBA has been a great boon to this franchise especially in attendance. And I tell you the truth, whenever we beat those teams, it makes me very happy."