Ever since my first visit in 1973, Big Bend has been my favorite national park. It has so much to offer... river, mountains, desert. The Rio Grande isn't powerful; on the contrary it's usually very peaceful and always scenic. The Chisos Mountains aren't the biggest or most awesome; they're simply beautiful and are a great addition to any photo. I live in the Sonoran Desert so the Chihuahuan won't ever be # 1 with me, but it just screams "Big Bend". It's barren and rugged and remote; you have to work to get there and survive there.
Big Bend is truly one of the crown jewels of the National Park Service.