have 2 suggestions that haven't been mentioned due to obscurity.
BFME2
It's like Starcraft/Warcraft on steroids. Take the constant clicking and make it faster. Never really went anywhere, but watching old demo files really shows the insane mechanics. Imagine Starcraft with 6 (or 7, depending on version) races, each with more units than starcraft, and with an even faster gameplay.
Civilization IV
The game is more of comparing players single player campaigns against each other with a normalized score than multiplayer head to head. That being said, the game is highly competitive. This year, 10 years after it's release had the biggest competition ever, Hall of Game Major Gauntlet 137, and it took 3 months to complete. The challenge is in the thinking as there is no technical skill involved, but only few can keep up with the best. There are probably 50 top players in a dedicated and practicing playerbase still in the tens of thousands, most of which can't even touch the top 200 players. Back in 2010 this game was even more jam-packed with players and a great competitive enviroment.
It's fucking hard because it's like hyperchess.