I love to do spell scrolls in a three different ways that sometimes overlap. One is spells that fill a gap in their list which is usually very similar to spells that they ask for. I encourage asking for things so I can know but don't give them everything. This is similar to spells that would be useful for the current adventure or the next one.

The other are weird or very situationally useful spells because I want to see them in play. These are just things that no one would ever pick because they are situational. These are just fun to have especially if they get them from NPCs or dead enemies. These I give out more freely

The last are thematically appropriate spells. Doing a cold themed dungeon get ready for lots of cold damage spells or other similar spells. These are kind of fun because you can get some odd balls in there as well.

I guess I love weird spells.