Learn Italian: I quartieri di Torino sono i meno rumorosi? - Are the neighborhoods of Turin the least noisy?

In Italian, you create the relative superlative (the most / the least) by putting a definite article before più (more) or meno (less). Here, the masculine plural article i (the) + meno (less) + rumorosi (noisy) creates "the least noisy" to match the subject I quartieri.

What happens if I forget the article i and just say sono meno rumorosi?

If you drop the i, you change the meaning from "the least noisy" to just "less noisy". The definite article is the magic ingredient that turns a comparison ("less than") into a superlative ("the least of all").

Quartieri ends in -i in the plural. Is the singular form quartiero?

No, the singular is il quartiere. It belongs to a group of Italian nouns that end in -e in the singular. These nouns always change their ending to -i in the plural, regardless of whether they are masculine (like this one) or feminine.

Why is it di Torino and not combined with an article like del Torino?

City names in Italian almost never take a definite article. Because there is no article before Torino, the preposition di (of) stays in its simple, uncombined form.