My opinion the National Army
The Royal Marines are part of the Royal Navy and nothing to do with the Army.
The Royal Navy first came into operation in the early 16th century but the British (or Royal Army) was set up in 1707. So therefore the Royal Navy is the oldest. Hope this help you! Addendum: The UK does not have a Royal Army. The navy and the air force are Royal, but the army is just the British Army. We do have various regiments and corps that are Royal though.
Lafayette referred to his army as the National Guard to emphasize its role as a citizen army representing the interests of the people rather than the monarchy. This choice of name reflected the revolutionary ideals of liberty, equality, and fraternity, promoting the idea of a national identity rooted in popular sovereignty. By distancing the force from the royal army, Lafayette aimed to foster loyalty to the nation rather than to the king, aligning with the democratic aspirations of the time.
Its based in the National Army Museum, Royal Hospital Road, Chelsea, London SW3 4HT
Royal Thai Army was created in 1874.
Royal Bhutan Army was created in 1950.
Royal Yugoslav Army was created in 1919.
Royal Cambodian Army was created in 1953.
Royal Bahraini Army was created in 1977.
Royal Army Ordnance Corps ended in 1993.
Royal Army Dental Corps was created in 1921.
Women's Royal Army Corps was created in 1949.