Someone who you can imagine living with for the next several decades. Shared values helps, although you don’t need the same opinion on everything, some kind of basic shared outlook is good. Sense of humour, dependability, good with kids (if you want kids.) someone who treats you well, and pulls their weight through harder times. Someone with a stable job. Doesn’t have to be rich, just stable.
Looks are nice, but no one is pretty forever and I’d rather be with someone kind, stable fun and ugly than someone hot and vacuous/awful/cruel.
avoid: cruel people, addicts, personality disorders, those who can’t hold down a job, and general deviants. Avoid anyone who doesn’t share your joint life tasks (men who won’t do housework, women who expect to be kept etc.) avoid anyone who treats staff of any type, or children or animals badly.
having seen friends divorce, the two main reasons have been infidelity and the slow buildup of small acts of behaviour. Twenty years of being treated like a housemaid is enough to kill any affection. Respect for each other and genuine affection will outlive most things. If you resent each other you’re doomed.