The algorithm mostly used in cryptography for secure data transmission is the Advanced Encryption Standard (AES). AES is widely adopted due to its strong security features and efficiency in encrypting data. It operates on fixed block sizes and supports different key lengths (128, 192, and 256 bits), making it suitable for a variety of applications. AES is utilized in many security protocols, including SSL/TLS for secure web communications, and its widespread implementation makes it a vital component of modern cryptographic practices.