This is probably the best place to start:

nostr:naddr1qq9hyetvv9uj6um9w36hqqg0waehxw309ahx7um5wghx6mmdqgsyvrp9u6p0mfur9dfdru3d853tx9mdjuhkphxuxgfwmryja7zsvhqrqsqqqa28837xa0

But I really do think the best discovery and learning happens when you try to run your own relay (it's not that hard at all). Also check out #Jumble which is probably the best relay explorer out there.