I actively study fossil birds so that's actually really hard for me to pick just one :') like... all of them

this one of Nahmavis from Green River has always been one of my favorites though

The modern landscape of the American west is but a far cry from its past. Early Eocene Green River formation preserves an incredible snapshot of time 50 million years ago. The locality of these fossils was once a huge lake system bordered by forests of sycamore and palmettos, and teeming with animals of all kinds, including birds, which were in the middle of an enormous radiation of forms.

Calciavis: A palaeognath which was a member of the lithornithids. Like them, it fed on insects from the forest floor and was a powerful flier. Some fossils preserve colour pigments of its feathers, suggesting this bird was a glossy black.

Limnofregata: The earliest known frigatebird that flew over the water just as its aeronaut descendants. Like modern frigates, it plucked prey from the water and the shorelines of the lake system.

Plesiocathartes: A bizarre bird that has had a complicated history. Now we know that it was an early relative of the cuckoo-roller from Madagascar and probably lived a similar lifestyle.

Celericolius: An early member of the mousebird family that had long wings and even longer tail. It may have flown at high speed through the forests.

Foro: This american bird is in fact an early member of turacos, birds which are unique to Africa today. Like them, it mostly fed on soft fruit.

Prefica: A strange bird who'se closest living relative is the oilbird of South America, which is a nocturnal frugivore. It could be that this ancient relative lived in a similar way.

Nahmavis: An early member of the shorebird family known from a spectacular fossil that preserves feathers.

Gallinuloides: The earliest known true galliform, this small fowl was among the early ancestors of guans, quails, turkeys and grouse.