The norm seems to be 1/12, atleast over here in Japan.
But I have also seen many kits in 1/8, 1/9, 1/6 and 1/24.
Personally, I would say that the subject, detail level and the skill of the model maker should decide on the scale used.
Tamiya does quiet a few 1/6 scale kits, IMHO, they are inferior to the 1/12 kits. It seems that each maker has a scale where they excel and others were they are inferior, may it be plastic, resin, vinyl, etc kits.