This is such a difficult question. My favorites are always going to be the 2 machines that needed the most repair work. They really shouldn't be working at all but I refused to give up: A 1930 Singer 101, and a 48-53 Necchi BU. They run like new... now. Both very smooth, and the loudest thing using them is the needle punching through material. A joy. Just a joy to use. I actually prefer the 101 over a 201, or 500a. Now, if I could dash back in the burning house and save 2 more... probably the Kenmore 117.740 and a 63' Necchi 525 (no FA).
What would 'complete' the collection? I would like to try a 201Kv2, yes I would, and a Mira to complete the early Necchi set (BU, Nova, Mira), and either a Kenmore 117.840 or White 651/666 to again complete the set (74-84-89)... but honestly... that's just the collector talking... I have enough. If I could find an early (3 drawer) type 40 DeLuxe Library table and a Necchi Parlor type cabinet... I might just call it quits. Maybe. I mean, well, you know, unless like a 55 Gritzner Zigzagger landed on my foot or a... a... no. No. No, I'd stop. Really. I would. I know I can. Maybe...