Uncharted Waters 2 New Horizons (SNES). It was a sandbox game long before the term existed. It's a great game that allows you to be an explorer, merchant, and/or a pirate. You also has six different characters to pick from and each had their own unique story. There was a nice selection of weapons, equipment, and trade items. You could use a ton of different style ships, from all over the world, and you could customize them to fit your needs. You could sail the seas with a single ship or make your own fleet. The game offered three different ways to fight off you enemies.
Seriously, it is one of the best games ever made and so few people have ever heard of it or even played it.
The most fun game? Tough one, so much to choose from but I'll go with Banjo-Kazooie. B-K is THE definite 3D Platforming game and not even SMG and SMG2 (which were great) are better or more fun than B-K.
The music, the witty dialogue and humour give the game so much charm and still makes it so much fun to play even to this day :)