For me one of the very early defining laptops is the Olivetti M15(+), because this already looks very common, compared to much later models. M15 is 512 kBytes 80c88 @ 4,77 MHz with CGA LCD, 2x 720kB Floppy. The plus version replaces one FDD by 20 MB XT-Bus-HDD.
Another important one is the first Apple power book, because it invents the mouse pad position in the middle in front of the keyboard, like 99% of the today's notebooks also use that layout.
But also very important is the Olivetti S16, S20, S25, D33 family (Triumph Adler Walkstation series, Acorn A4) as they were the first ones to have a mousepad, between keyboard and display. They are of the 286, 386SX-16 to 386DX-33 era.
Interesting detail: M15, S25 and D33 and Walkstation 386 have the courious detail that you can take out their keyboard and lay it in front of the laptop as external keyboard.
An interesting one is also the ATARI ST-Book as it introduced the "Vector pad" to move the mouse. That Vector pad works the same way as the red track point of later Thinkpads, but it's outside of the keyboard.
The godfather of every Netbook and other modern small formfactor notebooks is the Olivetti Quaderno, with NEC V40 CPU and ROM based MS-DOS 5. And if you look for even smaller devices, there is Atari Portfolio, Poquet-PC, Sharp PC-3x00, HP 200LX etc.
Don't forget Toshiba T1x00 as the first LCD based MS-DOS laptops.