I use 50Pin-SCSI-1-cable, the ones with the centronics plug.
Enough pins even for I-PAC + OptiPac and some cables reserved for other stuff you might need!
Plus, the centronicsplugs are easy-pluggable in my opinion.
Unfortunately the wires aren't big enough, maybe enough for screwing them in the I-Pac, but not big enough for quickconnectors you need for your joysticks and buttons. I'll solder them in a little PCB (you can get them readymade with all the holes and copperlines that connect 3 holes together), so I can use thicker wires on the other side.
Still have to do that (I used a IDE-cable like flat plug before, but they are just to hard to disconnect), but when I made it I might take some photos of it all put together.