I think it depends on where you're coming from—like, what do you already read? Romance? Sci-fi? Fantasy? Contemporary adventure fiction? Young adult (Harry Potter, The Hunger Games, et c.)?

There are a couple kinds of classics: there are adventure stories that have endured, and there are books that are considered outstanding literature. There's some overlap in these categories, but there are plenty which are definitely in one category but not the other.

There are some difficulties that accompany classics: 1) quality of translation, if not in a language you can read; 2) "old" language, if in one you can (also a problem in translations, actually, especially public-domain ones), 3) they take place in times that are alien enough that they're harder to understand now than they were originally, and 4) in the case of "literary" works, what one may be expected to appreciate about them is... not always the same sort of thing one may expect to appreciate about, say, a Tom Clancy novel. For most people it takes time to really get that, and begin to enjoy it.

Research and advice can help with #1, and one must get used to the idea that one may read a book once and hate it, but read it again in another translation and love it, and anyway, it's not a problem for everything, just works in translation. Exposure and practice are what get you through #2 and #4, precisely like getting into a new genre of music (especially one that's considered hard to appreciate, like, say, jazz or classical music, but hell, hip-hop's dense AF and takes time to really get, too). #3, exposure helps (as you pick up more about various cultures and time periods) but you've really gotta get comfortable with annotated editions and seeking extra material outside the book you're trying to read.

So, I'd say entry-level depends on the kind of classic, and where you're coming from.

Adventure-classic? For a native English speaker? Easy recommendation for King Solomon's Mines. It's great, it's pulpy, it's not terribly long, it's... weirdly modern in some ways? The language isn't old enough to be hard to read. Delightful, even in 2021.

Adventure-classic + literature? The Odyssey. Easy. If you find verse hard to read, just get a prose translation. Bonus: try to appreciate the so-called "Telemachy" (a few "books" [chapters] about Telemachus looking for his father, but mostly just drinking with his dad's old acquaintances). You'll be on your way to appreciating the more literary-side of classics if you can find enjoyment in that part.

You a sci-fi reader and want to learn to appreciate more literary things? Bradbury. Lots. Of. Bradbury. When you're loving his "boring" stories, you've made it.

Everyone who likes romance seems to get into Austen just fine, so that's easy. If you bounce off the more popular ones, try Persuasion. It's very short and, I gather, didn't get a "polishing" edit/rewrite pass from Austen so it reads differently from the others. Maybe stretch into Isak Dinesen's short stories? Not romance, exactly, but I think it'd do the job.

I'm garbage at fantasy, so I'll leave that rec. to someone else. Tolkien seems obvious, but IDK, and I'm not sure he's really a great gateway to broader "literature", and anyway I'm not that well read in him (I've read Fellowship and The Hobbit—I enjoyed The Hobbit). Ditto young-adult. The usual path there is just to move into friendly, short, easy-to-read semi-literary authors like Vonnegut, I think.