depends what you're really asking. if your asking which teams were the best, that's one thing. if you're asking which teams dominated their era the most (which to me is the definition of a dynasty), then that's a different question.
bird's celts were not a dynasty, imo. they were a great team. they never repeated, they won 3 titles, and they were the 2nd best team of their era. that doesn't meet my definition of a dynasty.
the '80's lakers were arguably a dynasty, although they only repeated once. their first 3 titles they weren't able to defend. not really a true dynasty.
the only true dynasty's the league has seen are, in order of dominance, imo, are russells celts (11 titles in 13 years), jordan's bulls (6 in 8, 2 3peats), and mikan's lakers (5 in 6 years).
you could make the argument, imo, that the 80's lakers and celtics were better teams, and would have beat the others, but you can't argue they were greater dynasties. the existence of both precluded each from being a true dynasty.