The rarest Apple portable is the Macintosh Portable. Technically, its not a PowerBook however its basically a Mac SE in a portable case.
The rarest notebook under the "PowerBook" moniker is most likely the A1138, aka the Dual Layer, High Resolution PowerBook G4 15". Its 17" A1139 brother was actually sold 6 months where as the 15" was only sold for 4 months. By the time those machines came out, the Intel switch was already announced so sales of PowerPC machines were already slowing (unfortunately), plus the very short life span is bad for replacement parts.