What is a good example of a transitional form that supports the theory of evolution

It is a truism in biology that every species that has ever lived on Earth, animal or plant or fungi, bacteria or protist, is either transitional, potentially transitional or went extinct without issue. The ones that normally get cited in lists such as these are the ones made spectacular only because their near neighbors above and below in the tree have not yet been discovered, so they look radically different from their nearest known neighbors in the tree, or they occupy a particularly critical juncture that we recognize in hindsight.

Pakicetus was a four-footed meat-eating ungulate who lived in Pakistan 50 million years ago. He looked like lots of carnivores before and since, but what makes him different is that he displays a mutation of the skull in the ear area, which we recognize as diagnostic of whales. So he doesn't look like a whale but is one; he is transitional. As far as water goes, he was very much in the same space as beavers are today. He was probably comfortable around water, but actually lived dry. His descendents, like Ambulocetus, which resembled an alligator, got more adept in the water, until, like Remingtoncetus, they no longer could move on land. All of these, and 20-odd more finds, are all transitional, moving towards our modern whales.

If you're looking for something along the lines of Archaeopteryx, Tiktaalik, the first "fishapod" is a good example of the transition from fish to land-dwelling vertebrates. There's a very good book written about called "Your Inner Fish" by Neil Shubin if you're interested in the subject.

Galapagos Turtles

Darwin's Finches