Ask HN: What's your favourite bot API/SDK?

Those of you who have implemented interactive chat/messaging bots - I'm curious to hear if there were any particularly positive/negative experiences you've had with existing APIs, SDKs or tools that you have used?

E.g. I know that Slack and Telegram have both had success with their APIs, but that doesn't necessarily mean developers enjoyed using them.

I'm looking to learn what API design patterns should be considered ideal, and which (if any) should be avoided.