Good paste tomatoes are seedless (or nearly so), meaty, and on the dry side―qualities that also tailor them perfectly to sauces and sun-drying. Some have good enough flavor to be used (and sold in supermarkets) as slicers.
'Roma' is the classic paste tomato, but in the tastings we attended, full-flavored 'San Marzano' and sweet 'Viva Italia' came out ahead. At 3 ½ inches long, 'San Marzano' is bigger than 'Roma' and has some resistance to late blight. 'Viva Italia' is a determinate type (it grows to a certain height, then stops), so all the fruit comes to harvest at about the same time. (That's a clear advantage for anyone who wants to do all their harvesting and canning on the same weekend.) And it's very sweet and disease-resistant. 'Principe Borghese' is particularly favored as a drying tomato.