"desireable" - for you or in general?
In General - Singers are desirable - there are plenty of collectors for the others, but in general, more people are interested in Singers.
Condition means a lot to value- there are a lot of machines with worn decals/finishes. In general, people will pay more for prettier looking machines
Availablity of supplies - in genereal people are more willing to buy a machine they can get needles and bobbins/shuttles for. There are some that if they don't come with bobbins - or a shuttle/bobbin case, the owner may have to pay more for those than they did for the machine. This is also why Singer is more desirable, supplies are available for most makes.
Now - if you want to get a amchine for personal use, then what makes it "desirable" is solely in your eyes. My husband and I have very different criteria on what makes a machine "desirable" - for him, price is a large factor, and for me, well, I've paid over $1000 to get a rare vintage machine I wanted.