If the Page is for business then I would recommend using your brand name. If not, it's a good idea to put a keyword or two in there. This is because people do find pages via the search engine within Facebook. And while it's pretty basic at the moment, it's getting more and more sophisticated. It's also getting used a lot more, too. (Then there's the fact that Facebook Pages are indexed by search engines. So you might be able to draw some extra likes that way.)
You also want it to be descriptive and easy to remember. Considering that it's prefaced by www.facebook.com it's a good idea to make the username reasonably short if possible so that the whole thing isn't too long.
Remember also that one of the best ways to get more likes is by interacting with other Pages like your own. And whenever you share a link, observation or comment the name (as opposed to username, which appears in the domain) is visible. Now if you have a descriptive keyword or two in there too it will pique the curiosity of some of that Page's fans and they'll have a look at yours. So you should give that a lot of thought as well.
For example, I have a political community page. It's right of centre, basically conservative and mostly focused on the Aussie issues I blog about. With these aspects in mind I took a branding angle for the username and chose MattHaydenBlogs. When it came to the actual name I decided on "Conservative Blogs AU". This is what will appear next to all my interactions with other Pages, so readers will be clear about what my perspective is. (BTW, I managed to make this change with only 17 likes so far. So it seems that the 25 like minimum is not set in stone.)