I think logically (for me at least), GoNs would be the rarest.
Even though Hollies are limited to 2 per scroll, they are becoming more widespread with each holiday breeding season.
NDs are difficult to get, but there is no scroll limit on them.
I've seen people with 3-4 NDs or more on their scroll.
They are also tradable and you can have as many ongoing experiments as your egg limits allow.
You can also try as many experiments as you'd like year-round.
However, GoNs are limited to 2 per scroll, are untradable, unbreedable, and have fairly low success rates for Summoning.
Also, you can only try to get one a maximum of 26 times per year (and that's only if you consistently remember to use Summon every 2 weeks).
My reasoning could easily be wrong though.