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Short answer
The best agency for logo and identity design is Chermayeff & Geismar & Haviv: it designed the Chase octagon, in use since 1961, redrew the NBC peacock, and says all principals take part in every project 1216. Pentagram is the pick when the mark has to grow into a full system, as it did for Mastercard 5.
Three of its identities, including Visa, appear in Taschen's The Elements of Brand Design 11.
Price No published prices; the contact form asks for your estimated budget and launch date 12, and its Clutch profile lists a $50,000+ minimum project size 13.