This’ll be a short one: there are only two RPG blogs I follow with any sort of regularity: Ars Ludi and Deeper in the Game. Both are a little more analytical/theoretical than other blogs I’ve tried, and regularly make me think. They’re the closest I’ve found to the sort of content I loved on rec.games.frp.advocacy back in the ‘90s.
There are many others over the years that I’ve read, but all the rest have fallen by the wayside—I just sort of stop reading them. I’ve recently been pointed at a few other sites, but haven’t gotten around to checking them out; maybe I’ll have a new favorite by this time next year.