The 23-year-old became Everton's joint all-time leading goalscorer in the Premier League with his 60th strike for the club against the Black Cats, drawing level with club legend Duncan Ferguson.
Lukaku now has 137 club goals during his fledgling career, and Williams believes that he deserves to be included in the discussion of the world's best players.
"He's top class. I would say he's world class. It's nice to watch him week-in week-out, and work with him every day," he told talkSPORT.
"His record speaks for itself, with so many goals at such a young age. He's a good guy to have around the dressing room - he's very popular. But as I say, I'm happy he's on my team, I'm enjoying working with him and I wish him all the best. I hope he keeps scoring for us.
"He's one of the best I'd say. Anyone who watches him and sees his finishing ability alone will put him up there."
Lukaku is the Premier League's joint top-scorer this season with 17 goals.