I think it depends on your goals.
If you want to play sessions, then a C whistle is yer pick. For those DDor, GDor, Am tunes, etc. as you say. The D/C combo is a standard session pair. After that, an E whistle may be most useful, as an easy “cheat” for A tunes, and the occasional E tune. And a low D whistle obviously gives you a tenor-pitched alternative for doubling with flutes, etc.
For personal playing, it’s whatever strikes your fancy. Eb and Bb are obvious choices, because they’re available as inexpensive Gens, Dixon Trad (Eb), some others. Eb is bright and nimble, favored by many; Bb is darker. In the mid-pitched instruments F and then A are prime. The other keys not so much.
I’d caution, though, against thinking about whistle pitches in terms of the tune keys - e.g., a G whistle for G tunes, etc. Most ITM tunes fit the D whistle range. So using low whistles (except D) gives you the wrong range, and requires octave folding. Normally you treat a low whistle as a transposing instrument and finger the tunes normally, resulting in a transposed tune for playing solo, or with other instruments or accompanists who can also transpose accordingly.
As for whistle as an assumed stepping stone to flute, or pipes, or ophicleide, or cowbell, that’s a different kettle of fish entirely. 
In sum, my personal order: D, C, E, low-D, Eb, low-F, Bb, A.
