Wikipedia is the best, if Wikipedia doesn't exists, Britannica would be the dominant.
You have to subscribe to Britannica, which might put a lot of people off. I did so for a while, in order to research stuff for university assignments.
Before Wikipedia, I recall using CD-ROM encyclopaedias (well, Encarta, but could have been others). Not as extensive as things like Wikipedia but what it did have, usually linked to other resources on the Web too so you could find out more without having to trundle through Google/(insert search engine here) search results.
Of course, minus the fact that some of the stuff is a bit tongue-in-cheek and is only ever composed of what people happen to be interested in, h2g2 was an early attempt at a Wikipedia-like "democratic" information resource. It might not have been so eclipsed without Wikipedia, and become more serious. In short, the idea was not originated in Wikipedia, and if it did not exist, there would be reason to create it...