Which Dec Emulation is the MOST useful and Versatile?

When you say "emulation" do you mean a software emulator like SIMH or E11? For those, the model choice is just a startup parameter, so you can change at will. Or do you mean an FPGA based one like PDP2011? There too the choice is a parameter, when you build the VHDL into the actual FPGA bits. In any case, if you want to pick a particular model, I would say 11/70 is a good choice. While near the end of the PDP11 era the Q-bus became mainstream, for much of the time the Unibus was either the only or at least the primary I/O bus. It has the full memory management unit and full floating point, so any software that requires these is happy. It has 22 bit addressing for big memory. And it is old enough that early operating systems like DOS will work. You could even turn on CIS instructions and call it an 11/74, the semi-mythical 11/70 variant for commercial applications (COBOL) that never shipped, some say because it was too good compared to the VAX 11/780. One more consideration: if by "emulator" you mean something in hardware that has an actual DEC I/O bus coming out of it and accepts real DEC cards, then a Q-bus system may be better, it depends on what I/O devices you can most readily find. If so, I'd go for the 11/73. paul