In uncountable terms the difference between ultimate and best
is that
ultimate is the sport of ultimate frisbee while
best is the supreme effort one can make, or has made.
As adjectives the difference between ultimate and best
is that
ultimate is final; last in a series while
best is superlative of good.
As nouns the difference between ultimate and best
is that
ultimate is the most basic or fundamental of a set of things while
best is the supreme effort one can make, or has made.
As an adverb best is
superlative of well POS=adverb.
As a verb best is
to surpass in skill or achievement.
As a proper noun Best is
{{surname}.