For me, this is the most impressive arch in Rome. It's located on a road between Caelian Hill and Palatine Hill where they once celebrated victory marches. It's 21 meters high, 26 meters wide, and more than 7 meters thick. It was built in 315AD to celebrate the 10th anniversary of the reign of Constantine and his victory over Maxentius at the Battle of the Milvian Bridge in 312AD. A restoration took place in 1804 and it is now decorated with medallions.
Constantine I is considered to be the first Christian Emperor since he converted to Christianity on his deathbed. This arch, however, makes no reference to this.