Hey Carol,
This is pretty much all personal preference.
My preferred keepers:
For Singer: Almost anything that ended in a 1 or a 3: 201, 301, 401, 403 (And Miriam may call me out publicly if I don't mention the 404.
) and the 500, which should have been a 501, and 503. I really like the German variants 411G, 431G, etc. 221 and 222s are fun little machines and great
light machines for piecing with a perfect straight stitch and quite collectible, so a -possible- investment if you can buy them at a good price. The 15s are work horses. The 201 is a work horse too, and quieter than the 15s, but some people have a harder time with free motion on them. Lots of people like the 66s / 99s, 127 /128s. I find them less desirable -to me- because the variable shuttle (127/128) is noisier and the 66s/99s, I guess it's just because they've given me the most grief for service, and they're less friendly due to the horizontal bobbin for what I do. (yes, I'm aware that they account for almost 1/3 of my collection, but 2 - 3 of them are also on the chopping block. Part of the herd that's getting thinned) That said, all of them make a great straight stitch, because they have a fixed needle bar.
For Pfaff - The 30/130/230, The 362-261(?) I have here would likely be robust if it hadn't been flagrantly abused before I got it, and I really like even the "still made in Germany" early 90s machines, even with the plastic in them. I haven't met a Pfaff yet that was in good running order that hasn't pleased me.
I'll take almost any German made machine in fact, regardless of the brand. I was in love with my Winselmann hand crank, My cousin has her now. Their bobbin winders, their look, and the ones with the big open hand wheel? ummm ummmm ummmm
Now this will make me unpopular: Bernina and Elna. Both of these brands but 1 of the Elnas I've had here (an SU) did not impress me once I got the panels off and started working on them. They were just so "fiddly" I've serviced about 7 between the 2 brands so far, one serger, and all left me feeling the same. Perhaps the service tech at one of the shops here said it best. " Something about the Swiss, It's like they get the machine working perfectly, and then go "OK, we need another... 6, maybe 12 more parts in here, with no change of functionality." That said, almost everyone I talk to who owns either brand loves them. So if you're using them, they're great. Servicing, which you'll likely end up doing when you get enough of the vintage machines, I think not so great.
I love love love servicing and using the Kenmores, especially the 158 series made by Jaguar / Maruzen. They're solid machines. Quieter on the whole than a 15, and nothing seems to really go wrong with them. The C877 series- I've never figured out who made them - often have that lovely retro look to them and are work horses as well.
Brother made some really cool looking retro type machines, but it seems they went early into plastic gears, especially that cam gear which is almost always cracked on the machines I've seen, and so I won't even take a post early 1950s brother anymore.
Some of the Japanese "clones" are so great! Again, nothing really goes wrong with them, and they'll sew forever. Some are also quieter than the 15.
Like Miriam said, all of them