The 1957 Singer 401 - I'm paper-piecing on it now.
The 15-91's from the 40's - I just finished quilting a vintage full/queen on one of them a few days ago and when they're not quilting, they're piecing. Never want to give those up, either.
The Kenmore 1802 and 1803 from the early 70's. If it weren't for the smaller area under the arm, the 1803 would be my most-used machine. It's the best free-motion quilter I've got, but the smaller space makes it more difficult to use for large quilts. The 1802 is a backup in case the 1803 breaks down and can't be repaired. Sews EVERYTHING. Quilts beautifully. Glides along like a Cadillac.
All the other machines, I could let them go if I had to. But, I hope I can always keep all of these.