The best domain name is one that is short, memorable, relevant, and easy to spell. Unfortunately, almost all those are already taken, at least if you want .com for your top level domain. If you find such a name, buy it immediately. Sometimes if you hesitate, you’ll tip off a domain watcher lurking to see what domains people are interested in. These people will grab up a site for $30 or so, and if you come back and decide you want it after all, suddenly it’s taken, and somehow “worth” several hundred dollars.
A long domain name that makes sense is better than a short domain name that doesn’t. Non-hyphenated domain names are better than hyphenated ones in most cases. If you can’t get the .com top-level domain, then .net is becoming a reasonable alternative. The .org extension is a good one, but some people think of sites with this extension as being nonprofits, even though they don’t have to be.
If possible, get a good domain name before starting your site and building your brand. It’s not that easy to go back and shoehorn your site name to fit your domain, or to shoehorn your domain into your sitename. It can be done, but the results are often less than satisfactory.