The iPad version of Love & Death: Bitten is what got me hooked on the genre. Still one of my faves. Great art, neat story and fun gameplay. The Awakening and Drawn series are two of the classics, if atiny bit dated. The Twisted Lands series is great if you like horror and are willing to use the hint button for navigation. The Dark Parables series is a fantastic take on fairy tales. Dark Tales riffs on Poe. Redemption Cemetery is popular. Grim Tales are top notch. Dark Strokes is a fun romantic one-shot. The Dana Knight series are relaxing and like a mildly supernatural Nancy Drew. Mystery Legends are oldies but goodies.
Edit : Almost forgot - Mystery Case Files and Mystery Trackers are the standard by which the other genre games are judged. Search for Ravenhearst.