Almost any of the DS Dragon Quest titles could have made this list, but Sentinels of the Starry Skies stands out as the best-received and first to reach international acclaim. The game sold more than 4 million copies in Japan and 1 million internationally in its first year of release, received a perfect score in Japanese gaming magazine Famitsu (only the 10th game to ever do so), and revolutionized the use of passerby connections in gaming, leading to advances in this function on the 3DS.
In fact, Dragon Quest IX is considered one of the catalysts for social interaction in gaming. Thousands of fans set up real-life meet-up locations, and eventually opened up a new destination in Akihabara (Tokyo's electronic district and anime mecca) in order to exchange special treasure maps.
Sentinels of the Starry Sky isn't just included on this list for its revolutionary multiplayer capabilities, though. The game features a 50+ hour story, challenging gameplay, character customization, and intriguing plot and characters that together make for a truly enjoyable experience.
The story follows a young Celestrian (angel) who falls from the heavenly Observatory down to the Protectorate below, the land of mortals. The main character must help out human beings in order to gain benevolence and earn their way back to the Observatory. The plot grows more complicated as it goes along, and eventually leads to the main character and their party of 3 human beings (fully customizable) on a mission to save both the mortal world and the heavens.
Gameplay is similar to other Dragon Quest titles, with the main difference being the significant increase in difficulty. Players engage in turned-based combat in order to defeat enemy monsters, stacking up damage by performing consecutive attacks of the same type. Different character classes (thief, priest, etc.) use different skills and attacks, and each character's class can be changed or upgraded when meeting certain conditions.
In all, Dragon Quest IX is an excellent RPG that effectively utilizes challenging combat, an engaging plot, and a quirky cast of characters in order to deliver a top-notch gaming experience that I would highly recommend to any RPG fan looking for a bit of a challenge.