This is kind of a tough question, because Disney doesn't 'require' reservations for its restaurants; there's no such list. However, certain restaurants are more popular than others, and can be extremely difficult to get into without a reservation. This especially applies to character meals, but many theme park restaurants, and certain resort restaurants as well.
To further complicate matters, some restaurants, particularly in theme parks, will sometimes turn away anyone who does not have a reservation - no waiting for a table allowed. However, that same restaurant, the next day, might be allowing walk-ups to wait for a table. There doesn't seem to be any particular pattern that I can figure out, it's probably based on staffing, time of day, and who knows what other variables that we're not aware of.
So, the only 'safe' thing to do is to make an ADR for any TS restaurants that you want to visit.