D2PRO9 I would think to be just because of jtag.
However D2Lite (the new chip they released) should be fairly futureproof as well. The chip itself ships blank and then its programmed via dvd, (hacked firmware for backups), so it works differently than most the other chips we've seen so far.
The wasabi2 and wiizard have both shown that via dvd you can add d2e support, so the spi can do quite a bit of updating as well.
If I had to rank
1. d2pro9 (jtag support)
2. D2lite/wasabi2/wiizard (alphabetical).
Provided nothing too drastic comes out I think all of the chips are pretty much considered futureproof.