Well I voted 15 mm but it depends.
1) I don't have any WW2 armies any more and have decided to not start the WW2 PTO forces I wanted to a few years ago, hence no 20 mm which is the local gold standard for WW2 non-skirmish battles.
2) My over all size preference for my prime interests is (Air Combat and mass battles is 3 mm which was not on the list or 6 mm,) which makes up 100% of the former (3 mm/Air) and (for 3 mm and 6 mm) over 50% of the post-1938 historical and SF figures I currently own.
3) In some eras/genres there literally is only one scale for what I game (Soldados and Persidials are a bit thin on the ground in any scale...)
4) 15 mm seems a good compromise but so far I only have Fantasy, SF, Supers, and VSF in that size. And there are miniatures sets in 25+ mm in those that I will keep anyway for skirmish/RPG oriented games. Plus all the legacy Der Kriegspieler Fantasy figures... sigh.
5) It does depend to a degree on what your local group is willing to play and the scale of your individual game plus all the "preferences" (rational or not) mentioned above.
Gracias,
Glenn