My favourite keyboard ever is the IBM Model M. Best switches ever.
But honestly "best" keyboard is a minefield fraught with fond memories of computing. Getting to spend hours standing in a Radio Shack clacking away on a Model II or Model III writing basic? Great keyboard. Getting to play Star Trek on an ASR-33 dialed into a CDC mainframe? GREAT keyboard. Getting to the computer lab first before anyone else and getting the TEK-4104? Oh man I can still feel those squishy keys and remember the screen flare from pressing clear. Great flipping keyboard.
Getting to the VT-52 with its buzzy relay buzz with every pressed key or the glorious quacking clack of the VT-100? GREAT KEYBOARDS. And of course getting my very own ADM 3 A terminal when I was learning UNIX? Ermagerd. I'll be alone in my bunk... with my keyboard.
Still love the Model M of course but with the rise of the cube farm they're no longer viable. I miss everyone having an office and quiet space to work far more than I miss any particular keyboard and as you can tell I really like my keyboards.