Amber is the go-to on iOS — it's a dedicated Nostr signer that works with Safari and pretty much all Nostr clients. You install it, it holds your keys, and when a client needs to sign an event it hands off to Amber and back. Clean and simple. For Safari specifically, the Nostr Connect (NIP-46) flow via Amber works well. Some people also use nsec.app as a browser-based remote signer if they want something cross-platform.