Ask HN: Best chat client for company use

I would like to introduce a chat client into our company. Writing mails and dealing with the overhead for something like "no, make it #3c3c3b" just slows everything down.

Right now I am probably the only one using the "fancy" "new" stuff like Slack and as I work for a quite traditional printing company it will need some time to get new things up and running.

Ease of use is a big point and it must run on almost everything (old Macs + Windows systems but also the newest of both worlds, tablet support would be great).

Now I wonder whether Slack is the way to go because it got channels, which I find great, or if there is something better.

I even thought about setting up an IRC server and using that, but... well... traditional company...

Any ideas on this?