strongest in flavor or in caffeine? With robusta you will get twice the caffeine vs arabica, but robusta taste pretty unpleasant. If you want to try robusta, Paradise Roasters have some top quality Indian 100% Robusta espresso. It's not for me, it's not recommanded for dripped. But assuming you are in US, it beats getting coffee from Australia.
If you want the strongest in flavor, brew any good freshly roasted coffee with more coffee and less water. Or try an Americano, better yet try a double espresso from a good coffee shop - now that's good stuff.