Aka Willow or Alder, but not sure which.
Compared to a Least Flycatcher, the eye ring is less distinct and the overall body shape is more long and slender, vs the much more compact, big-headed appearance of a Least.
beyond Willow/Alder, it’s largely speculation. I’m personally leaning toward Alder personally due to the greenish hues to the upperparts, rounded head, etc. However maybe it’s an eastern Willow and I’m just used to western Willows. Anyway yeah probably not identifiable to species with confidence but I agree with Aidan