izigame.me

An interesting curio to say the least, Vulgus (released in 1984) is Capcom's first video game.
It's a very barebones space-themed shmup, with no distinct levels but rather alternating backgrounds until you reach an enemy that sort of acts like the final boss. Then, the game repeats with incresed difficulty until you die.
While almost prehistoric by today's standards (you only get a shoot button and a cannon button to fire a piercing projectile) and with utter disregard for anything resembling a difficulty curve, I'd say that Vulgus is pretty fun in its simplicity. Then there is also the fun that comes from playing the part of video game archeologist and discovering mama Capcom's first game ever!