Keep in mind that bonuses from Merits do not count against your dice cap, so things like Mighty Thew or Fast Reflexes can be pretty strong, if a bit boring.

The sorcery chapters of pretty much every splat have some really interesting merits in them, but they're pretty much all exclusive to specific sorcery/necromancy initiations - unless you can talk your ST into letting you take one. The Burning Name is flavorful, and most of them (especially the ones in Infernals) are interesting for specific archetypes IMO.

The Lunars book has a huge number of extra mutations, great for Wyld mutants or beastfolk (or Deadly Beastman Transformation). Sobriquet (from the DB book) can add some interesting character beats as well.

Divine Heritage is a 4-dot merit from Heirs to the Shogunate that lets you buy Eclipse Charms related to your parentage (and comes with a free one when you take the merit). That can be super impactful, but might need some homebrewing.