If I had to pick just one favorite Batman artist (and who would want to have to do that?) I'd likely go with Jim Aparo.
Aparo was the longtime artist on the Batman team-up book, The Brave and the Bold, which was a great bang-for-your-buck (or 25-cents) when I was a kid. Not only did you get Batman, but a guest star from the wider DC Universe. Along with fabulous Aparo art, you'd generally get a cool script from Zany Bob Haney, known for his sometimes wildly imaginative/loopy plots.
Aparo was an incredibly prolific and talented artist. He took the realistic Neal Adams-style Batman and made it his own, oftentimes doing his own inks and lettering to boot.
What captivates about his art, I think, is the intensity. The faces of Aparo's characters express so much emotion with their furrowed brows and anguished mouths and those little "emotion action" lines emitting from them (take a look at any Aparo-illustrated story and you'll get what I mean).
His story-telling and panel flow was so smooth -- a lost art today when many artists are great at drawing amazing pictures, but poor at portraying action and helping guide readers through the story.
Aparo was so prolific that he got taken for granted. He was one of the great artists of his era, but we all got so used to his frequent, excellent work that we didn't give him as much credit as some of less-productive peers.
That's changing as more and more of his work is reprinted and we can see his great art not only on stories featuring Batman, but also the Phantom, Aquaman, Phantom Stranger and the Spectre.
DC published a hardcover collection of his Batman work, "Legends of the Dark Knight: Jim Aparo Vol. 1," earlier this year.
Here are some samples of Aparo's Batman work: