It's quite hard to tell...But top SEO blogs have some things in common. Lee Odden's What makes a Top SEO Blog? has tackled the salient points:

So, what are the best or top SEO blogs? It really depends on your definition. This IS a popularity contest, but its also a relevancy contest. The top ranked blogs don't always satisfy readers' information needs or interests. Sometimes its a matter of style and persona that drive readers to more niche and specific blogs.

What makes a SEO blog a leader? To start, I think there's no better metric of popularity than RSS subscribers. Link measurement and PageRank measurements are either too unreliable or flawed. Technoratis method of counting links is a step in the right direction compared to what Yahoo Site Explorer reports, but there does need to be a better way to qualify link sources.

I also think its important to note that just because a blog has a lot of RSS subscribers, links, high PageRank, posts frequently, etc does not mean its the best blog for you as an information source. That's a personal decision made by you. The sorting and ranking of blogs by itself should not solely dictate what blogs are the best sources of your information.

You need to actually read a good number of those blogs and the blogs they link out to in order to find the right information neighborhood that's right for you. If I had to pick just one search marketing related blog to recommend to someone that wants to become familiar with the SEO/SEM industry, it would probably