In Knowledge, Reality, and Value, Mike Huemer sets out to write the world's best introduction to philosophy. I haven't read enough of his competitors to know for sure if he succeeds, but I can say he sets the bar extremely high and it would be hard to beat.
The book is clear and easy to read. In fact, it's so clear that it serves as a test. If you can't handle this book, philosophy is not for you.
Lots of philosophers hide their bad arguments, and sometimes even good arguments, behind bad writing and obscure prose. Huemer cuts right through it all and gets to what's important and what's worth thinking about.
Huemer doesn't hold back from telling which positions he thinks are correct and why, but he is also fair to other side(s) and lets you know when he's taking a minority position.
While most intro books (I've seen) focus heavily on the history of thought for the sake of the history of thought, Huemer instead examines topics that a normal person might and should care about, and looks at arguments because they might help us figure out the truth, not because some old person said them.
Perhaps the best part of this book, if you were to assign it to students, is that it does not merely teach them what philosophers have said and why. It helps teach them how to do philosophy and do it well. Huemer not only includes helpful guides about writing essays, making arguments, and avoiding getting stuck on semantic bullshit (see, e.g., the "democracy is an essentially contested concept" junk that ruins so much democratic theory), but leads by example. You get to see him reason through hard topics, including when he manages to reframe debates that people get stuck on.
Further, many intro textbooks make it seem like philosophy is kind of waste of time. Here are hard questions. Here are some answers. None of the answers work. We've made no progress. Guess you should've taken a different class to fill out your gen eds, no? But Huemer has none of that. He makes it clear why these questions are important, why smart people might get stuck on them, but then in the end helps you see that yes, we've learned something.
I highly recommend this book, regardless of whether you're new to philosophy or a soon-to-retire professional. You'll learn something and have fun along the way.