A common English expression takes the form of "It was one of, if not the, best experience[s] of my life." In this expression, what is the correct number for "experience[s]"?
This can't really have a correct answer. To take your example: "It was one of, if not the, best experience[s] of my life."
'It was one of' requires one of a number, so calls for: 'best experiences', however, the alternative: 'if not the' only calls for a singular, which would be: 'best experience', and thus we have a contradiction. Which do we choose?
Doing a quick search online, the choices seem pretty evenly spread:
Gain Weight...Lose Your Mate by Carla Tuner
"...it was one of if not the happiest and most promising day of our lives."
USC has faced its share of adversity this season by Michael Vega
"...it was one of, if not the best job in America."
Is this the best own goal you'll ever see in your lifetime? Step forward Milan Gajic... by John Drayton
"...it was one of, if not the sweetest strike the 27-year-old has hit during his career."
However, as is seen in these two, the 'if not the' is actually rather appended:
Team Louisiana Lacrosse: Player Bio - Donald Davis
"It was one of (if not) the first tournaments we competed in..."
Wimp2Warrior: Testimonials - Adam Helmrich
"...it was one of (if not THE most) rewarding things I have ever done."
This would suggest to me that one should opt for the plural, but I feel the best construction here is the one presented by Donald Davis in his team bio.
Usually, with the usage of two commas to connote an "interruptive" set of words in the sentence, the sentence should also make sense with the interruptive phrase separated by the commas out of the sentence completely. By this notion, the word "experience" should be plural, as the word "of" should be matched with "experiences."