Blackberry. I have never had a more efficient handheld for doing what I need to do most, emali and chat/text messages. The version with the thumbwheel, enabled me to race through messages. I would expect slack to do great on it as well. It was not very good for web, apps, games, like my current smartphone is, but I think my current smartphone is clumsy and slow when it comes to message and email.
Old school. I don't think theres enough demand out there to bring it back tbh. Even though there are still many passionate ex-users - and maybe current users. Touch screens have proven to be far superior as they serve both input and output purposes.
The problem ends up being you don't know where to touch without looking at it. Imagine having a touchscreen keyboard on your laptop. You'd have to look down to verify where you were.