Forget Tiktaalik. As a notable transitional fossil, I nominate the H. sapiens from Jebel Irhoud, Morocco.
Neanderthal and all previous hominins had an elongated skull and a large face, compared to the smaller face of H. sapiens. The skull from Jebel Irhoud, dated around 300,000 years ago, has a small face, like us, but an elongated skull, like Neanderthal. Additionally, all H. sapiens infants are born with an elongated skull, like Neanderthal and every previous hominin, but the rapid growth of the infant sapiens’ parietal lobe, cerebellum, and frontal pole reshapes the skull into our distinctive globular pattern by the first birthday.
From the abstract of the linked article:
We identified a mosaic of features including facial, mandibular and dental morphology that aligns the Jebel Irhoud material with early or recent anatomically modern humans and more primitive neurocranial and endocranial morphology.