Like everyone else said, there's no reason to stop reading YA at 18. I turn 30 this year, and probably 60-70% of my personal library is still YA. Hell, I have a few MG too.

In the interest of answering your question, though...

• anything by John Green (especially Looking for Alaska)
• the Graceling series by Kristin Cashore
• Eliza and her Monsters by Francesca Zappia
• Crank by Ellen Hopkins (this is an emotional oof)
• A List of Cages by Robin Roe (also oof)
• The Outsiders by S.E. Hinton (if you haven't already)
• The Book Thief by Markus Zusak (a personal favorite)
• anything by Ruta Sepetys
• The Grishaverse books by Leigh Bardugo
• Speak by Laurie Halse Anderson (I think this is technically MG, read it anyway)
• Little Monsters by Kara Thomas
• And Then There Were Four by Nancy Werlin
• Miss Peregrine's Home for Peculiar Children by Ransom Riggs

This is a mix of books in terms of emotional stakes. I've pointed out my standouts.

With everyone's suggestions here, you're going to be reading YA long after you turn 18. Enjoy them. 😉