Learn Russian: Это формула, которая гарантирует отличный уход. - This is a formula that guarantees excellent care.

The noun уход actually has two distinct meanings. It can mean "departure" (from уходить), but it is also the standard word for "care" or "maintenance" (like skin care or looking after a patient). In this second sense, it is tied to the related verb ухаживать (to care for / to look after).

Why is которая in the nominative case? Isn't the formula what we are talking about, making it an object?

The case of который is always determined by its grammatical role inside its own relative clause, not by its role in the main sentence. In the clause "которая гарантирует..." (which guarantees...), the word "which" is the subject actively doing the guaranteeing. Therefore, it takes the nominative case. It is feminine singular simply to match its antecedent, формула.

Is the verb гарантирует imperfective or perfective?

Гарантировать is a bi-aspectual verb, meaning the exact same word can function as either imperfective or perfective depending on the context. In this sentence, it acts as an imperfective verb because it describes a general, ongoing characteristic of the formula.

Why don't the words отличный уход change their endings, given that they are the direct object of the verb?

They are indeed in the accusative case here! However, уход is an inanimate masculine noun. In Russian, masculine inanimate nouns (and the adjectives describing them) in the accusative case look exactly identical to their nominative forms. If it were an animate noun, the ending would look like the genitive.