My first 6502 bringup on a breadboard was just a CPU and ROM (no RAM) with A9'H'A9'e'A9'l'A9'l'A9'o' while it was attached to a logic analyser so I could see if it was running. Shortly after I got ram working and added a UART, but it was the easiest instruction to see something I recognised and keep the PC moving forward through memory (Of course NOP is good too, but then I don't know if reading is working right or if I'm just reading the same thing over-and-over).

Also my assembler wasn't working, so I had to enter that directly in the TL866 ROM programmer's hex editor & didn't remember any other opcodes.