I'd say Mystery Dungeon is the best for me,but I have a couple of reasons why you should buy Pokemon Ranger.
Pokemon Ranger is very fun.You get to save people and doodle on the Touch Screen.Although Im stuck on Mission 7,my friends have shown me what Mission 10 is like:fun.It has a change of music,if I can remember clearly.
Mystery Dungeon is also very fun.It potrays edge-of your-seat events,such as the encounter with Ninetales and Gardevoir.Even though most not-your-choice rescues are hard,they may seem fun in a while.If you're buying this game,Totodile is a good choice as your partner.