The "Greatest" ... that's really a very subjective term.
I've been a Celtic fan for over 40 years, (yes, I know that dates me, haha), and I've been lucky enough to witness first-hand most of the series on this list, and
all of them by at least secondary means. There is something special that stands out for me about each one, but while the term "greatest" probably means to most here either the closest, or most competitive, what I immediately think of when I hear the term is "sweetest victory".
I have to say, (and this will most likely outrage some, or at least leave them wondering about my sanity), that the sweetest victory, and hence the greatest series to me, was the 2008 Finals against the Lakers. To go from such a horrible season one year, and then put a team together in such a short time that results in a World Championship the very
next year is absolutely amazing.
It was also following one of the longest, driest periods in the history of the franchise, and what could prove to be the culmination of the career of Paul Pierce, a guy who went vastly under-rated for most of his time in the NBA, and carried the franchise on his shoulders, with very little complaining, and very few instances of wanting to be traded ... in other words, a loyal Celtic.
Not only that, but there were two other players added that year who had gone through very similar circumstances and dry spells. KG had paid a lot of dues in Minnesota to no avail, and Ray had come up short at his previous digs as well. These guys were thrown together in a very short period of time for the sole purpose of winning it all, and they did ... in their very first year together! They sacrificed their stardom, set aside their differences and massive egos, and committed to the Ubuntu spirit, all for the sake of the ultimate goal.
Now also add the fact that they were being led in the Point Guard position by a guy who had never been a regular starter in the NBA, and was only in his second year at this level. I don't think there have been many other instances where a 2nd Year PG was asked to lead a team and did so to the point of winning the O'Brien trophy in his first year starting, if any. That in itself is amazing, and another element to this awesome story.
Along with the rest, this was also a time when the Finals had the incredible odds of being a contest between the two most storied franchises, and the two most bitter rivals in the history of the NBA. The chances of that happening along with all the other elements I've noted, is just incredibly rare. And not only did we beat the Lakers on our own home court in the last game, but we absolutely
humiliated them, and our MVP, The Truth, completely shut Kobe DOWN!
We did it with as much defense, as much heart, as much selflessness and sacrifice, and as much Celtic pride as has ever been displayed in Boston, all during a day and age when such team spirit and cooperation is rarely seen or practiced. I can honestly say that Red would have been proud ... and as proud as he
ever was ... and it would have made the scene complete to see him light up that cigar one more time, (laws notwithstanding

)
All these things may not seem like a huge deal in and of
themselves, but put them all together and it makes for an absolutely amazing series, and one that to
me, is easily as satisfying, as exciting, as jubilant, as sweet, and as
great, as any other I've seen.