Surprised I haven't seen a thread like this before (maybe I missed it, given the limitations of flyertalk's search engine).
To me, the most "useful" blog is the one that alerts me to deals I can use, and otherwise informs me of what's "going on" in the frequent flyer world without having to read every flyertalk post.
I also like blogs that are interesting, amusing, and well written.
I'd rank my current blog preferences as follows:
1. ViewFromTheWing.com
2. ThePointsGuy.com
3. boardingarea.com/blogs/loyaltytraveler
4. FreeFrequentFlyermiles.com
5. OnlineTravelReview.com
6. TheFrugalTravelGuy.com
7. boardingarea.com/blogs/onemileatatime
There are some other worthwhile blogs that aren't exactly "frequent flyer" blogs, but have useful travel information and deals. Crankyflier.com is a must read for me, and I'm impressed by the money-saving focus of dansdeals.com, even though most of his non-travel deals have little relevance to me.
What are your favorites?