This is NOT about the top-5 players from the Celtics and Lakers. It is about the best all-time, 5-man squad in their prime that can be theoretically assembled and play great as a unit.
Celtics:
PG: Dennis Johnson - 6'4" two-way player that could score and distribute the ball. Great teammate. Jo Jo White would fit here, also.
SG: John Havlicek - the man's play spoke for itself. I have nothing to add.
SF: Larry Bird - who else?