i have a set of predefine camera position and look that switches when a button is clicked,

It defaults with the first position, and when switches it in sequential manner, right now, ill just set the position and look directly without any animation or transition.

whats the most effective way and smooth to transitions between these positions and look?

there is LERP and SLERP to consider

The most smooth transition is a curve, which moves the camera to the final position over time. In Unity for example I was able to utilize the Animation class for that but as you didn't add Unity in your tags, you need to calculate that by yourself