There's really nothing to make here: you either have good tomatoes, basil, mozzarella and olive oil or you don't. Any one of these four legs of the stool go missing and it all falls down.
For this I used three different types of heirloom tomatoes (even the Roma's were an heirloom variety). Any good farmers market will have them this time of year; I got mine at Whole Foods since the damned Dupont Farmers Market is open on Sunday and not Saturday, which is when I went shopping. At least I tried to go local.
Buffalo mozzarella works best for this dish but if you can't find it choose your best cow's milk variety. Cut the cheese fairly thick and let sit on a cutting board for about 5 minutes to let residual liquid drain off. Cut the tomatoes to the same thickness as the cheese and stack them how you please. Sprinkle JUST A LITTLE flake-style sea salt over the tomatoes, drizzle with extra virgin olive oil, and garnish with whole basil leaves.
These flavors carry their own. This is one dish were less is, in fact, more.