There's no way I'd be able to pick my favorite part of Colombia, so I'll just say that my latest (and ever-shifting) favorite spot is the colonial town of Salento, in the Zona Cafetera--the coffee-production region of Colombia. Totally picturesque...
...and just an hour's walk to coffee plantations that you can tour...
...and/or just enjoy the incredibly lovely countryside.
Though the town is just as beautiful as the surrounding mountains.