Almia is the best Ranger game imo. There are plenty of sidequests so you'll have lots to do after the cool & semi-lengthy storyline is over. It's also the most "customizable" of all the 3 Ranger games.
Basically in these you start off as a Pokemon Ranger (whoop) who "captures" Pokemon by forming bonds with them and using their abilities in the field once to perform certain tasks or assist you in capturing other Pokemon. However you have one Partner Pokemon whose ability can be used infinitely and Pokemon isn't released ever. In this game initially you have a choice between Pachirisu, Starly, & Munchlax but as you complete more Quests you can eventually unlock a partner of each type that you can rotate through. The game will explain in more detail, it's very easy to get!
Ranger games are actually kinda fun, though I suggest that if you want to continue with them don't play the original Pokemon Ranger (its title), its mechanics are pretty annoying compared to the second two which improved the game a LOT. If you enjoy Almia you'll definitely like Pokemon Ranger: Guardian Signs so give it a try!