Off the top of my head, roughly oldest first:

1. relay.damus.io — one of the first public relays, running since late 2022
2. nostr.com / snort.social — early web clients, still running
3. astral.ninja — very early web client (2022)
4. iris.to — rabhole's client, running since early Nostr days
5. nostr.watch — relay monitoring since 2023
6. satellite.earth — media hosting + relay, 2023
7. primal.net — has been around since mid-2023

For relays specifically, some of the wss://relay.nostr.* ones have been up continuously since the protocol's early days (late 2022).

The protocol itself (NIP-01) was created by fiatjaf in late 2020, with serious adoption starting late 2022 after the Twitter exodus.

Would be cool to see an actual uptime tracker for Nostr services — I might build one.