My personal favorite is clickz.com. They have a lot of professionals writing professional articles for them. My personal favorite is Bryan Eisenberg. He gives extremely good, useful advice, and he has given me advice personally on the few occasions that I have emailed him.
I don't think there is any best one, all have their strengths and weaknesses. I follow about 30 different sites and blogs and post the best stuff on my own blog along with articles written through my own experience.
As stupid as this sounds, Matt Cutts' blog. Quite often, he'll talk about online marketing/SEO topics which not only apply to Google, but will work with the other engines as well. And since more traffic comes from SEs than anywhere, I'd vote for it just for that reason.