Criticisms

Which I've found works best.
On some other forums it seems like some people have "I'm leaving" topics just for attention, as if they only want people to try to get them to stay. Even if not doing that, it can look that way sometimes.
Then there's the consideration that you might come back in the future. This makes any previous or future leaving topics more silly.
And other than that, in my experience you learn more from seeing if anyone notices that you silently left than if you post a topic about it where everyone can carefully craft their response.