The best 68k 'Book is the 550c... though they're pretty rare. It's basically an upgraded version of the 540c with a full '040, a larger (10.4") screen and a black case that was only sold in Japan:
http://68kmla.org/wiki/Macintosh_PowerBook_550c
Apart from that, both the 190 and 540 are pretty nice... and should be a lot easier to find.