Stem-amniotes are pretty obviously transitional between fish and amphibians-they still have lateral lines like fish; but have necks, lungs, and a number of other amphibian-like features; a dentition unlike anything else; and the wrong number of toes for anything modern.
Rostroconchs show a pretty obvious transition between something like the early-Cambrian “monoplacophorans” and scaphopods.
Ancestors of kingdoms are going to be old enough that finding them is all but impossible, let alone recognizing them.
Most ancestors of phyla would look like generic worms, and are thus all but impossible to identify. Examples of distinctive, very basal members of phyla seem to include radiodont arthropods and myllokunmingiids.
An obvious example for transition to a class is the long series of non-mammal synapsids.
For orders, the series of fossils intermediate between generic cetungulates and modern cetaceans come to mind.
And an obvious case for transition to a family would be Echinofulgur and the busyconids. And given that Busyconidae seems to fall out closest to Fasciolariidae on genetics, Echinofulgur makes perfect sense as a transition.