If not used often. Put ontop of the fridge

Ours is a built in above the wall oven. My wife is on the short side and ours has the pull down door. If we could do it over again we would do a swing door as she has a hard time reaching over the door to place items in/remove them.

Ours sits in one of those microwave cabinets under a regular cabinet. There is a plug directly behind it. Doesn't take up much room and looks good.

A lot of new houses around here put a 20 amp outlet in the pantry for the microwave. Keeps it out of view and you don't have a built in taking up prettier cabinet face space. Simpler replacement too when it comes time to swap it out.