From a cooks perspective, I think it should be "espresso", or "caffè" not "coffee" in any good tiramisu recipe, which in turn would already suggest a type of beans and roast.
Note that the Italian term caffè denotes the strong black liquid you get from an esspesso machine or a moka.
The coffee in a tiramisu has to be concentrated to supply enough flavour without soaking the ladyfingers or other bisquits used. (The same principle applies when you use a liquor.)
When choosing the beans /the roast, the same rule as for wine applies: Use what you drink. And seriously, unless you are making an insane amount of tiramisu, you will be using only very little of one pack of coffee.
I use whatever I choose for my Espresso (if I feel rich, I'll splurge on something from my local roaster, in a pinch the usual suspects like Illy or Lavazza, sometimes even a Starbucks Espresso roast cough) and call it a day.
So in sort, get a classic Italian espresso roast, prepare it authentically and you should be fine. In a pinch, use another coffee, but make sure it is very strong. You'll need a few tablespoons tops for a standard tiramisu recipe.