I've got a pretty good Least lean, but don't see this as being a photo to be getting definitive from.
How are we ruling out Yellow-bellied? Personally, I find the primary projection uncompelling, almost intermediate between the two. Eye ring is not quite as bold as I like on the Least's locally. The belly not only looks yellowish, but the sides and flank dark, and for a long way down. I like the contrast between throat and breast for Least, but not the lack of contrast between malar and throat.
I'm not saying it is a Yellow-bellied, particularly with the color of the wing-bars on this juvenile, but if there is just a smidge of a color cast problem (and the dullness of the green in the leaves have me wondering about a minor blue cast), that could change things. So a Least lean, but I sure can't get definitive on this particular Empidonax from this one photo.