Char Aznable from Mobile Suit Gundam.
To go in detail on why, Char is a character who constantly changes as the series goes on. Being one of the first iconic anti-heroes/anti-villains in anime, he is easily the most interesting character in the original series by having a pretty strong revenge plot in which he plays both sides very well whilst showing his genuine care towards characters such as his sister and Lalah. The prequel OVA series, The Origin, as well as the manga it's based on help further show his rise as we learn about his childhood and how willing he is to do despicable acts to achieve his otherwise sympathetic goal makes him even more engaging.
In Zeta Gundam, he takes on the interesting role of becoming a mentor figure for the protagonist Kamille whilst also learning some things from Kamille during his rare instances of mistakes. What's most interesting, whilst brief, is seeing his interactions with Amuro and how at this point, they're on a thin line between friends and enemies, of disdain and respect by not fully letting go of the past, yet still looking at the bigger picture. It's a shame this only lasted a few episodes, as it's easily the best part of Zeta. His cool factor is also off the charts.
Char's Counterattack is where he fully becomes a villain. Many would cite his goal in it as out of character, but that's only if not looking between the lines. With the genuine care he had for Kamille and knowing what the war ended up doing to his pupil, Char taking drastic measures to bring about change in what he perceived to be for the greater good of humanity. He also shows some subtle regret and self-hatred of the action he must take with the line "Amuro, I'm about to do something extremely wicked. If you're nearby, sense my presence." It suggests that Char, to some extent, wanted to be countered by Amuro, be it for his own desire to settle their rivalry or feeling some subconscious level of regret in the actions he was about to take. The rivalry he had with Amuro up to that point and how their roles had changed so much throughout the series until that point really made the movie exciting as we see the conclusion to it.
9/10
I would've cited Griffith as the best, however, if it weren't for how he lost all nuance after becoming Femto and just became an evil rapist who can only ever speak about "muh dream".