My favorite is shift + alt + up/down arrow to move to the next unread channel. Works between servers too. Super easy way to knock out hundreds of unread messages at once.
Ctrl+K just searches for channel. I was looking more for something where I could switch between 2 different channels quickly. Like I'm messaging someone but then have to quickly switch to a different channel to address some issue and then quickly switch back without having to click on or search for the previous channel.
For what it's worth you can also Ctrl/Cmd-T and search for someone to begin DMing them.