Blender Secrets - Modeling Holes
If you want to avoid n-gons and boolean artefacts, modeling with quads is the way to go. First you need to add some more edge loops. For a round hole, you don't actually need that many loops. 3 edge loops per side of this cube will do.
Press Ctrl+R and press 3 to increase the number to 3, or scoll the mousewheel.
Now select 4 faces in the middle and inset them by pressing i and moving the mouse.
Right-click and from the LoopTools menu choose Circle. (LoopTools add-on needs to be activated in Preferences.)
Inset the selection again slightly and extrude down by pressing E and Z, to create some support loops.
Extrude further down and create some support loops there as well, if you like. Now we've got a hole with perfect quad-topology.
