I posted a bunch of great examples in an earlier thread somewhere. I don’t have a particular favorite, but I also don’t think that examples of speciation are a good way to deal with the topic. The basic steps in speciation are so obvious and so undoubtable that a person should be able to understand how speciation is not only something that can happen, but something that must happen. Someone who doubts speciation is someone who doesn’t even know what it is, and so showing them examples won’t accomplish anything without an understanding of why it has to happen, and that requires a patient explanation of what it actually is.
In other words, confronted with a “speciation skeptic,” I would outline allopatric speciation (meaning I would discuss geographical isolation, in lay terms), then outline reproductive isolation (easy to understand), then listen for what still confuses them. In my experience, one will then hear some combination of 1) nonsensical platonic confusion in one form or another; 2) redirection to “macroevolution” in some way; or 3) a grateful response to some new knowledge. If I hear response 3, I can then excitedly overstay my welcome by talking about sympatric speciation and ring species and gene flow and genetic diversity and and and and… oh wow, look at the time.
[Edit to head off potential confusion: speciation must happen in general, because reproductive isolation of populations happens all the time. That’s not to say that speciation must happen every time two populations are separated—there are all sorts of reasons why it may not occur in separated populations.]