Tough call, because even the supplements I didn't personally like were pretty good, although less than perfectly edited (Ground Vehicle Guide, Ships of the French Arm spring to mind).
Given I quite rapidly tired of killing bugs, my interests drifted away from Kafer material, so I would have to say my shortlist would be Nyotekundu, Aurore (was anybody else freaked for several months before realising that it was the planet from Decision at Hunder Rift, the Battletech Novel?) and, despite it's flaws, Bayern. These, to my mind, caught the sesne of wonder aspect of good hard SF without over reliance on military plot lines. The OMS Andrew Carnegie was a good friend, even turning up in my weird cyberthulhu game in the Sol system Belt.