My favorite is my Singer 66 treadle. I'm like a kid with their favorite toy when I sew with it. My other machines excel at certain task so depending on the task that one is my favorite.
The one machine that I would like to have is a metal geared vintage that can also do stretch stitches. It would replace the vintage 1990's plastic machine in a case that I picked up for $20. It works ok but it's not an everyday work horse. I haven't done an active search for a replacement since I rarely need the stretch stitch function.