Of course I will tell you the obvious in here. The ideal was to contain the best search term for the market you’re trying to enter; and the shorter and more generic the best.
However and as we all know that’s almost impossible today. Well you still have options available. Avoid hyphen and .info domains. Google considers these as spam footprints more and more. Try .net .org or even .biz. Are you on a non English market? Then try local country tld or IDN domains.
Deleted domains were once a nice place to go also. However, most SE now devaluate totally the ranking earned by previous owners, so you’re starting from zero as a new domain.
I will have a lot more posts on this topic. So keep watching.