I just a few days ago started playing Guardian Signs, the most recently released Pokemon Ranger game (2010) and I've been enjoying it. I first tried Shadows of Almia, but found it a bit slow to start, so I switched to GS and I think it gets going a bit quicker: you can have multiple quests going at once, instead of only one at a time and seven pokemon friends right from the start instead of three.
My impression of the gameplay so far is that they're light action rpg with the battles centered around drawing loops with the stylus to convey friendship to the pokemon so they can tag along with you to use a field ability to get rid of some obstacle of some sort. When you battle a pokemon using the stylus, they have all sorts of attacks ranging from slashing at the stylus to chucking rocks or loosing clouds of poison gas depending on the Pokemon type (Pokemon only have one type in this, e.g golbat is poison).
You are a pokemon ranger who goes out on missions to help pokemon and people in trouble, and in GS foil the pokemon pinchers who want to steal pokemon and control them with special gauntlets.
One thing I very much like is that you can ride on Eneti and Raikou, and later you can ride on Suicune and Latias/Latios apparantly. Seeing all the different sprites of pokemon from all sorts of different angles is also interesting too. Some of the sprites, like Mightyena, IMO could be better, but some of them, like Enti and Raikou look really good.
The music is quite nice, this is the music for riding on Entei etc:
https://www.youtube.com/watch?v=HFUIw_fiGSw 