First hit on Google was this list made by some Redditor: https://www.reddit.com/r/Betamax/comments/z8cun4/films_that_only_exist_on_betamax_or_alt_editions/
In any case: the big advantage of Betamax (and VHS) was how easy it was to get home-recordings done. An entire generation of family photos / family events (birthday parties, etc. etc.) are on Betamax and likely lost.
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