Say's Phoebe is a grayer bird than this one, has almost no crest and has an almost-black tail. I agree that the warmth of the underbelly is tempting, but I think on this bird it's due more to the warm light of the late afternoon/early morning, or during a smoky-hazy sky (which we're getting a lot of here in AZ this year). Say's Phoebe also has obviously darker lores, sometimes almost looking like a bit of a mask through the eye, which contrasts more sharply with the throat and breast, which is a very light gray. When you brighten this photo up, you can see how much more relatively light the underbelly appears, no dark lores, and still that brown overall cast.
Willow Flycatcher is a much smaller, less robust bird than this one, and particularly compared to the beak on this bird.
I'm certainly not willing to go to the mat for either Ash-throated or Brown-crested without hearing them, but I'm still leaning toward Brown-crested this morning. For what that's worth.