On DD the biggest Know-it-alls are Ozoneocean, Bravo1102 and Isukun.
Interesting list, but I can tell you there is one name on there that doesn't surprise me in the least… ;)
Personally, I break my “know-it-all's” into two categories: credentialed know-it-all's and non-credentialed know-it-all's.
You probably wouldn't want to take my advice on the non-credentialed know-it-all's. I have a feeling I'm even more immature about it than ayes due to the fact that I end up making enemies with them. I can't help it though, when I'm talking to them I tend to end up mocking them. And it's worse because know-it-all's are slow to pick up on that sort of thing (everything is taken seriously). In the end, it just turns into a giant ragefest, which you don't want.
Credentialed know-it-all's however? I've run into hundreds of them in college. The only thing you can do there is to stay quiet and nod your head. Seriously, their mind is in a whole other plane of existence and you'd be powerless to fight them let alone know how to get there.