Second the X4. In the Fairwheel Stem test, it was bested only by the Pro Sprint stems. The older 3T with the four faceplate bolts also tested well.
From my perspective, the older Easton EA90 (with the full faceplate) also seems pretty stiff and is a solid design - bolt the top two face place bolts all the way then torque just the bottom two. I believe it comes in a 6 or 10. It looks like they redesigned it though, so not sure how stiff the new version is.