I remember reading a report about someone's experiences and benchmarking of various systems on the 1400 and the general consensus was that System 7.6 was the best compromise of speed and functionality. Certainly 7.6.1 works fine on my 1400cs, it boots quickly, doesn't use much RAM and does everything I need it to. Really depends on your requirements though as OS 8.6 may be better suited for more modern and demanding applications (with upgraded RAM).