Some of the bosses in Smash Bros.' Subspace Emissary mode are amazing. Check out the screens below and you can see Rayquaza from Pokemon and Metroid's Meta Ridley.
Rayquaza is a legendary Pokemon from Emerald who attacks with his charge and his mighty Iron Tail.
Porky is from Earthbround where he was called Pokey Minch. He's in two forms here - in his mech and as a statue. When he's in his mech you'll have to watch out for his little machine legs if you want to survive. He also has a guided laser and can launch Porky robots. As for Porky's statue, he's very powerful and you can't beat him so you just have to dodge and shield.
Then there's Ridley, the commander of the Space Pirate Army from the Metroid series. He flies around in the air, descending to hit you. Meta Ridley is another form of Ridley from Metroid Prime and the fight against him takes place on the back of the Falcon Flyer. He has attacks where he scrapes his feet along the ground and pushes down on the Falcon Flyer, sending you, erm, flying.
The other two bosses mentioned here - Duon and Galleom - are unique to Smash Bros. Galleom is an iron giant who you have to fight twice. He can transform into a tank and fire missiles.
Duon is a tough character with guns on one side and a sword on the other so you'll have to dodge his big blade and guided missiles.