You most definitely want to avoid PETG as it is pretty... flexible...
PLA is the way (the MPCNC, for example, is designed with PLA in mind) in this case, as with ABS you most likely need higher printing temps, bed temps, an enclosure to keep even the slightest drafts out...
PLA is actually the stiffest of those materials, just keep it cool enough that it doesn't warp! I expect that would only be a problem around the spindle which can get quite hot, and maybe stepper motor mounts.
