Both sequels have their good points, for different reasons.
Part II: the multiple time-travel paradoxes and changes to the timeline fascinated me to no end. Stuff like that (alternate universes, huge changes that can result from one tiny alteration in the past) falls into my "special interest" category.
Part III: satisfying conclusion to the whole thing (Doc meets the love of his life--in another time--what does that say about him?) and a very uplifting "moral": THE FUTURE IS WHAT YOU MAKE OF IT.
Loved the entire trilogy. Own it, watch the movies quite frequently.
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The existence of the leader who is wise
is barely known to those he leads.
He acts without unnecessary speech,
so that the people say,
'It happened of its own accord.' -Tao Te Ching, Verse 17