Again, the wording of today's challenge is ambiguous. I'm not sure if I'm supposed to choose from among my favorite books in the romance genre (you know, the smut section) or my favorite book of any genre that contains romance, so I'll do both.
My favorite romantic book would be Pride and Prejudice by Jane Austen (surprise, surprise). The romance between Elizabeth and Mr. Darcy is pretty much perfect. The banter between them, the misunderstandings, the grand gesture, which result in one of the best literary love confessions of all time. The ending always has me grinning like an idiot. There's also a bit of a Cinderella story going on, and a man who changes because of his love for a woman. (Note to impressionable young readers: This very rarely happens in real life. Do not expect a guy to change for you. It's a fantasy.)
It's been a while since I've read a true romance novel (from the smut section), but I'd say my favorite is a tie between Thunder and Roses by Mary Jo Putney and Something Wonderful by Judith McNaught. I haven't read them in years (I went through a romance novel phase back in college) but I remember really enjoying them both.
Honorable Mentions: Gone with the Wind by Margaret Mitchell (non-smut) and The Duke by Gaelen Foley (smut).