Personally, I think Ridley is the best Nintendo villain ever. And there are a couple of reasons why:
1: He is easily the coolest looking villain of Nintendo. I mean, look at him, he's a ' dragon. And dragons are already awesome. And that appearance is pretty intimidating...
2: Unlike Bowser and Ganondorf (Even though I love those villains too), Ridley has a relationship with Samus even before the series started or before she even started her journey. He raided Samus's home planet and murdered her mother right in front of her. I mean, wow!
3: He is the most efficient villain. Rather than take over a planet, Ridley, in spite of being a Space Pirate commander leaves that job to his forces and instead decides to prevent Samus from completing her missions. Many of his battles are downright amazing (With the battle in Prime being one of the, or maybe the greatest battle in a Nintendo game). His battles are engaging, atmospheric, equal and challenging.