What can I use as a counter argument?
There is not one evidence you can use to convince deniers. There are many that put together, they back up the theory.
Richard Dawkins had tackled this very question in his book The Greatest Show On Earth. I have summarised my understanding in my book The Tyranny of God: Liberating Ourselves...
But if your looking for something quick, I tell people that disputing fossils is an old religious argument against evolution. No longer do we need to rely on fossils to back track who our ancestors were. With new advances in our knowledge and technology, the evidence for evolution is inside each and every one of us. It is in our blood. Our DNA. Our DNA shows who and what our ancestors were: that not only can we trace ours to other people in the planet (the technology exists but may still be expensive for most people), we can also see that we share common ancestors with our cousins, the primates, the mammals, the reptiles and fish.
ThE truth is reflected in our bodies: our tail bone which is a remnant of our tails as primates, our arm skeleton we inherited from fish as they walked on land, or skin organs (skin, hair, sweat glands and teeth) from our reptiles and mammal ancestors. Our full colour vision that our primate ancestors evolved... And so much more. There's a documentary on this : Your Inner Fish