Perhaps the most challenging game on this list, the sequel to Dragon Quest Monsters Joker, and the fifth game in the Dragon Quest Monsters spin-off series, Dragon Quest Monsters Joker 2 vastly improves the gameplay of its predecessor.
The Dragon Quest Monsters series has always had a strong focus on breeding, even before Pokémon had the option. Breeding powerful monsters takes a lot of trial and error to get what you want, due to having very few rules. Because of the detailed breeding system, catching every new monster you encounter is obligatory if you want to create a monster with the best stats and skills. Monsters may also have hidden bonuses that won't be obvious to new players at first.
Another factor that motivates catching many monsters is the difficulty of the game. Sometimes you won't be able to progress to the next part of the game until you have a monster strong enough or have reached a high enough rank. The game expects you to stay on top of everything, so if you aren't properly prepared for an especially challenging battle, you may have a difficult time getting past it.
Featuring a more challenging difficulty than most other monster collecting RPGs and an extensive post-game, Dragon Quest Monsters Joker 2 is great for any monster collecting RPG or Dragon Quest fan looking for a new challenge.
Hopefully the Nintendo 3DS sequel, Dragon Quest Monsters Joker 3, will be announced for Western release soon.