Many people love Darth Maul, as he was a fantastic villain in Episode I. You don't hear much of his story in the movie, but the cool thing is that Star Wars has a lot of text about him to help you know a lot more.
He was seemingly going to
be a great Sith from the beginning. Maul went to the Academy of Orsis to become an assassin that could help the Sith in the future. The idea was that if he could be a great assassin, they could send him on missions that would allow them to stay hidden. This was vastly important, as the Sith were just starting to rise again at the time. They needed a skilled assassin and Maul was perfect.
He was told to not focus on the Force, but rather focus on heightening his senses, which would be a great thing for Maul since it made him a better assassin (to have heightened senses in a world where he would need them far more than the Force). However, he was not without Force knowledge, which only made him more of a danger to go up against. He didn't enjoy using the Force as much as physical combat, which is where he excelled.
Darth Sidious trained Maul intensely and forced him to be able to withstand a ton of pain. It seemed he had all the knowledge Sidious gave him on top of the sheer talent he possessed, making him a force to be reckoned with, one that no one wanted to cross. It surely helped him when fighting Qui-Gon Jinn, whom he killed. He was known as the first man we really saw with the dual lightsaber. Although he was not the only Sith to use it, we happened to see him as the first due to The Phantom Menace.
Darth Maul gets overlooked on lists like these, but he deserves a big nod.