The Lakers have been the most consistently outstanding franchise in NBA history.
Since there's no longer a huge disparity between the number of championships each team has won, I'd say that the Lakers are the better overall franchise.
The Lakers won 5 titles in their six seasons in the late 40s and 50s... the Celtics were not really a championship contender until they acquired Bill Russell... and then they became the greatest dynasty in the history of ANY pro sport in the U.S.
When the Celtics won their 16th championship in 1986, they led the Lakers in overall titles by a count of 16 to 9.
After that, the Lakers won 5 titles without the Celtics winning one, and they closed the gap to 16-14... The Celtics now lead 17-15.
Prior to 1960, the Lakers won 5 titles and the Celtics won 2.
During the 60s, the Celtics won 9 out of 10 titles (76ers won the other).
The Celtics added two more titles (to the Lakers one) during the 1970s.
It was during the decade of the 80s that the Lakers became a better team, and beat Boston two out of three in head to head finals... The Lakers also became the first team to repeat as champions in 19 years (since the 69 Celtics).
Since failing to make the playoffs in 1975 and 1976, the Lakers have failed to make the playoffs only twice (1994 and 2005). During that same period, the Celtics have failed to make the playoffs ELEVEN times, including six straight years from 1996 to 2001. It's really during this period that the Lakers have caught them as a franchise....
Head to head of course, the Celtics won 9 of the 11 finals meetings, including the 2008 finals. The first seven of those victories involved Bill Russell-led Celtic teams.
But I'd say the Lakers have a slight edge only because of the terrible run the Celtics had in the 90s... even though the Lakers didn't win a title in the 90s, they at least were a playoff team almost every year.
The Celtics had more all time greats who began their careers in Boston (and most of whom never left), while the Lakers acquired several of their all time great players, such as Kareem, Shaq and Wilt.
Both Russell and Magic played their entire careers with their original team, although both draft picks were acquired via trades, as were the draft picks which became James Worthy and Kevin McHale.
If you break it down by decades, you'd get:
1950s - Lakers
1960s - Celtics (by far)
1970s - Celtics
1980s - Lakers
1990s - Lakers
2000s - Lakers (despite the 2008 finals, Lakers still have 3 titles this decade, and still have shot to make it 4)
Of course, it's understating it to say that the Celtics were better in the 60s.
But if you're talking the total history from 1940s through today, I'd give a slight, slight edge to the Lakers because I think they've been more consistently good. And the Lakers haven't had a stretch like the Celtics did starting in the mid 90s...that was just bad.
btw, the Lakers have only missed the playoffs 5 times in their history, and only once did they miss the playoffs in back to back seasons. The only times they missed the playoffs were 1958, 1975, 1976, 1994, and 2005. The Celtics have missed the playoffs 16 times, including those 6 in a row I mentioned earlier.
Finally, the Lakers also have the best regular season winning percentage in NBA history.
The Lakers are the only NBA franchise with an all time winning percentage over .600 (they're at .617). The Celtics are second at .594. The Lakers are 2970-1841 (.617) and the Celtics are 2922-1999 (.594)
The number of down years the Celtics have had more than makes up for the two additional NBA titles they've won... The Lakers are the most consistently excellent team in NBA history.