I like how you've phrased the question. I think that there's a difference between 'favourite book' and 'favourite read': I may never reread some of my favourites (looking at you, Infinite Jest) but there are a lot of good books that I just love to read over and over.

The Phantom Tollbooth is one of my favourite reads. It's written for children so it's quick and manageable, but it's also very thoughtful, and it constantly puns and plays on its own words. A lot of common phrases of speech are illustrated literally (this is as great as you'd expect).

I've read the Lord of the Rings trilogy countless times as well. It's a long journey of a series, with lots of twists and sidequests, but the only time it ever loses or bores me is in the mountains at the very beginning of the Return of the King. That's a short section in three long fascinating books.

Like OP, I need to underline when I read now. It keeps me focused on the book. It also forces me to re-read, to slow down and appreciate the words. This is mostly true of long/dense books; the first novel I seriously marked was Ulysses. Some books I can't mark because there would be too much underlining (The English Patient), but in that case I also don't have trouble focusing.

I can't mark a book with a perfectly clean spine. But I love finding underlining and marginalia in used books, and I generally try to buy used.

For some of the books I love, I have two copies: one marked, one clean (a loaner). The only thing that bothers me about underlining and marginalia (now that I've gotten over my knee-jerk need to keep books clean) is embarrassment if I lend books I marked on years ago, because I was an idiot.