Hector: Badge of Carnage is the best adventure game on iOS, period. You need it if you're any sort of adventure fan.
Scarlett and the Spark of Life: Episode 1 is a hilarious, Monkey Island-style adventure, but only a couple hours long. Still, it's only two bucks last time I checked.
City of Secrets is apparently really good, but I haven't tried it yet.
Return to Mysterious Island and its sequel are good if you're looking for something more serious.
Papa Sangre may not be exactly what you're thinking of when you think of "adventure games," but it's one of the most intriguing adventure games I've ever played, period.
Ace Attorney, a port of the DS game, is worthwhile if you've never played it.