For me it start with feedback from relay users in the first place. I really like the concept how Coracle has integrated reviews for relays for example. This is done with notes kind 1985 (a label) from 

After this feedback compoment comes the performance (which are aggregated on 

https://nostr.watch

 for example). You could combine those two aspects to decide if the relay is giving a 

nostr

How can we compare nostr relay performance?

inscitia

"what a `good service` stands for?"

For me it start with feedback from relay users in the first place. I really like the concept how Coracle has integrated reviews for relays for example. This is done with notes kind 1985 (a label) from [NIP-32](https://github.com/nostr-protocol/nips/blob/master/32.md). 

https://imgprxy.stacker.news/E3QYn3ubMciveOnCC-8sD2TdT65-TyOdzvC3R0R-Hf4/rs:fit:600:500:0/g:no/aHR0cHM6Ly9zaGFyZXMuc2ViYXN0aXguZGV2L1FMWmtLNTJQLnBuZw

After this feedback compoment comes the performance (which are aggregated on https://nostr.watch for example). You could combine those two aspects to decide if the relay is giving a `good service`.