The Cartier Tank, firstly introduced in 1917 by Louis Cartier, is an all time favourite of mine. It represents classic style and sophistication without being over the top. The rectangular case and the roman numerals give really mesh together into a work of art.
Personnally, I think this watch can be worn in all occasions. Throw it under a nice suit or just with a plain white t-shirt and a military jacket. There is no way to go wrong with a classic Tank!