For me . . .it's the "Free Hand System" on my Bernina. It's a lever attached to the front of my machine and aims downward so that my upper leg controls the lifting and lowering of the presser foot. This enables me to turn and manipulate the fabric as I sew without taking my hands off the fabric. I absolutely couldn't sew without it.
Secondly . . .and I know many who find this machine gadget a bother . . .I couldn't live without my needle threader. Didn't think it would be that essential 'til I bought my first machine with one and now . . .I probably wouldn't be able to thread the needle otherwise LOL