Rubi idaei fructus is a source of nutritionally important bioactive chemical compounds, mainly antioxidants, which strengthen the immune system and can be used in the prophylaxis and adjuvant therapies of many oxidative stress-induced diseases. There are no literature reports presenting a comprehensive comparative analysis of the antioxidant activity and nutritionally relevant metabolites contained in the fruits of repeat-fruiting raspberry cultivars, which are commonly grown in Europe. The aim of this study was to carry out a comparative analysis of the antioxidant potential (Folin–Ciocalteu, DPPH, FRAP), the content of selected primary and secondary metabolites, and the qualitative and quantitative composition of amino acids and fatty acids in the fruits of R. idaeus cv. ‘Pokusa’, ‘Polana’, and ‘Polka’. The fruits of the analyzed cultivars have a low caloric value (171–219 kcal/100 g); low content of available carbohydrates (6–6.6%) and total carbohydrates (3.4–4.8%); and high levels of dietary fiber (4.7–5.8%), vitamin C (22.8–27 mg/100 g), anthocyanins (25.1–29.6 mg/100 g), and flavonoids (0.5–2.6 mg/100 g). The fruits were found to contain valuable unsaturated fatty acids (35–60%), especially MUFAs with dominant oleic, elaidic, palmitic, and erucic acids and PUFAs (α-linolenic, eicosapentaenoic, and linoleic acids). MUFAs from the ω-9 group accounted for 12–18%, whereas the content of PUFAs from the ω-3 and ω-6 groups was in the range of 15–23 and 6–21%, respectively. Exogenous amino acids, accounting for 56–62%, were dominated by leucine, phenylalanine, and lysine. The following order of the total polyphenolic content was established in the fresh fruit juice from the analyzed cultivars: ‘Pokusa’ < ‘Polana’ < ‘Polka’. The different antioxidant capacity assays used in the study confirmed the high antioxidant potential of the fruits and fresh juice from the three R. idaeus cultivars. This indicates that raspberry fruits can serve as a source of nutrients and can be used as a valuable supplement in a healthy human diet and a raw material in the pharmaceutical and cosmetic industries.