Having first watched Farrell in the 2000s in Alexander I cannot believe I am saying this today but such has been his transformation over the years. He has surpassed Leonardo Dicaprio (who is doing far fewer films these days) & Christian Bale for me as the actor whose work I am the most interested in.
Back in the day you could say that Farrell would have faded out after a while because he was never a massive draw despite doing mainstream films. But in the late 2000s with films such as In Bruges he started doing off beat stuff as a leading man. And he was at ease at playing second fiddle or secondary villains.
In the 2010s he has shown such a variety of range that I don't feel any other lead actor mainstream actor has matched. Consider his hilarious secondary villain in Horrible Bosses, his weird roles in Yorgos Lanthimos films Killing of a Sacred Deer or The Lobster, Seven Psychopaths, supporting act in The Gentleman or the incredible work in The Banshees of Inisherin just last year.
This is not all. Farrell has also been on TV as the lead in True Detective S2 where he was great and even better as a sinister villain in the highly underrated show The North Water that came out in 2021.
Speaking of villains, one cannot forget his work as The Penguin in The Batman where he clearly stole the show in a stacked cast of actors. And deservedly so he is getting his own spinoff next year.
There are tons of other weird movies he is a part of and he elevates them each time. He is an actor who continues to refine and develop his art and as a result has become one of the most interesting actors working in Hollywood today.
EDIT:
Cannot forget his work as Grindelwald in Fantastic Beasts. That same character was played by accomplished actors like Johnny Depp adn mads Mikkelsen but Farrell was way ahead than both in that role.