I'm moving up from a set of Panasonic SD cameras to HD, but the biggest question is, which ones are better suited to FCP?

What's been your experience? AVCHD? HDV? h.264?

Out of the three you mentioned, H.264 is not an editing format. It's a delivery format.

Other HD formats that also work well are XDCam EX (Sony EX1 & EX3), XDCam, DVCProHD (Panasonic P2).