Let's say I wanted to compare the relative levels of inluence of two bloggers in the same field. Although no method other than direct access to their visitor stats is going to be entirely accurate, there are several pieces of data that can provide a clue as to how "far and wide" a blogger's readership extends:

• Alexa data - though inaccurate, unless it's being spoofed, it will show relative levels of traffic in the same sector with some degree of accuracy.
• Bloglines subscribers - not everyone's demographics is hooked on Bloglines, but again, when comparing apples and apples, it's good data.
• Del.icio.us bookmarks - blogs get bookmarked quite a bit, though sometimes a single post or two might have more taggers than the blog as a whole. For the purposes of measuring influence, it's best to just analyze the blog homepage numbers.
• Technorati's link count data - inside the blogosphere, it's actually fairly good, though skewing definitely occurs in some sectors and much of the blogosphere-external links and readership goes unobserved.
• Yahoo!'s link count - a fairly good way to see how many links total the blog has to it (note - scroll too the last page of Yahoo!'s displayed results for more accurate link number data)

Using these, we might come up with a sample table for relative levels of influence in the SEO sphere... I'll ask someone here at SEOmoz to do the research and try to have it posted tomorrow afternoon - just leave comments with which blogs you'd like us to compare. My initial thoughts are:

• Mike Grehan (and possibly his ClickZ column, too)
• Threadwatch
• SEOBook
• Matt Cutts
• SearchEngineWatch
• SERoundtable
• Philipp Lenssen (GG Blogoscoped)
• V7N Blog

Let me know who else you'd like to see compared. Maybe we can put together a good weighting formula and crown a "biggest influencer" in SEO title... which gets me thinking - we could take bets on who we think will take it. From the list so far, I'd probably go with Matt or possibly Blogoscoped.