Unown is a Pokemon from Gold/Silver that has 28 different forms. Each form is a letter of the alphabet, plus a question mark and exclamation point. Unown is special because it was originally only available at physical events. And at those events, only specific letters were available. Over time, it's become easier to catch at least one form of Unown, but certain letters have only been available a few times.

So, which letters are the most common? Which ones are rare? And where have they been released?

I've been keeping track on this page: /apps/pokemon/unown

This page shows the data in 4 ways:
1. It shows which ones I'm missing and which events they were available at.
2. It shows an ascending list of the rarest Unowns and their events from the last 3 years.
3. It shows an ordered list of how many times each Letter has been available, and which events.
4. And, it shows each individual event where Unown had been available, and which ones were there.

I use this page to determine whether or not I should throw away some Unown after I catch them. If I go to an event in Japan, and a particular form appears that has not appeared back in the US, I'll hold on to more than usual so I can trade with people.

The Exclamation Point, for example, was available at Go Fest in 2019, but has not been available since.