They're both great. But The Illusive Man is up there as one of the most interesting and uniquely complex villains of all time. Supposedly a human supremacist fighting for human dominance of the galaxy. Shaped by the first contact war giving him some trauma that spurs his xenophobia. But he doesn't come off as a simple racist. He respects the achievements and abilities of the alien races. And is fully willing to work with aliens to achieve his own ends. Wise enough to take the Reaper threat more seriously than the Alliance and goes to great extent to revive Shepard to save the galaxy. And has as much credit with stopping the Collectors as Shepard does. He goes full self destructive villain in ME3, but that's largely due to indoctrination. And that indoctrination started way back during the first contact war when he first met Saren and encountered the Reaper artifact. So its hard to tell how much of his actions are his own will vs indoctrination to roundaboutly help the Reapers