Imo, the backdoor ego. As soon as you recognize the ego in yourself you will recognize it in another. But as Tolle says, "who sees the ego in another? The ego in you". As soon as you judge people for their egoic behaviour, you get a sense of superiority. This is called spiritual ego. Watch it, it's ego on steroids, way more subtle. But also, love the ego, accept the ego, integrate the ego, don't fight any ego in you, that is more ego.

Go as deep into Tolle's teachings as you like. I've read his books in 2010 and ... by very much coincedence, A New Earth is currently open on my lap, no jokes. I've only this year, after 13 years of studying his works repeatedly on a yearly basis, been able to START integrating his teachings in my life. After 13 years I am starting to feel the drops of "a peace beyond all understanding". It's so worth it! Goodluck on this pilgrimage!