Championship Efficiency Rating = (Championships Won / Number of Seasons Played) - Average Probability Of Winning Championship
The average probability of winning championship changes considerably across the franchises given the huge expansion in the NBA; the probability of winning is much less if there are 30 teams compared to if there are only 8 (which there was in the '50s).
The results are not that surprising for those who know their NBA history; the Boston Celtics and Los Angeles Lakers are streets ahead. (For brevity's sake I've cut out all the defunct franchises from the list below, many of which only played a season or two, although of course they were included when calculating the ratings.) What is more interesting is that only six current teams out of 30 have a positive rating, and one of those just barely. If that doesn't illustrate the 'competitive imbalance' problem in the NBA then nothing does.
LA Lakers 0.187
San Antonio 0.073
Golden State -0.018
Oklahoma City -0.021
New York -0.033
New Orleans -0.035
New Jersey -0.038
LA Clippers -0.041
Note too that Atlanta and Sacramento, despite winning one championship each, come out on bottom, since they played a lot of their histories in a relatively small league with a higher (theoretical) probability of winning.