I have only been able to play one of the Ranger games, Guardian Signs, but it's my favourite Pokemon game of them all.
The battle system is improved from the previous Rangers, I hear.
Overworld encounters with Pokemon is much better than tall grass.
Recruiting Pokemon to help you clear real-world elemental obstacles is the best.
The story is engaging, as are the missions and locations and puzzles, which is simply something I don't find in the other Pokemon series.
The only weak points are the side missions in the past, which are frustrating, and the fact that your screen WILL get circular scratches on it over time.