The storyline for PMD2 is enticing, but the dungeons and Special Episodes for sky can be insane. E. G. Temporal Spire, Spacial Rift, Deep Dark Crater (only because of Cresselia, she's like 1,000,000,000,000,000,000,000,000,000 (that's an octillion for you math freaks) levels lower than me and has the stats of a level 60. >.> PMD1 is pretty much the opposite. The dungeons are easy (Buried Relic is almost as easy as Tiny Woods to a level 75 with a LOT of apples and a monster-house-killing move.) the storyline is mediocre, and let me just say I don't like Lugia (he one shotted me after I cleared a 98 floor dungeon(Silver Trench or something for those who care)) .
PMD3 hasn't come out in the US yet, but it seems similar to an upgraded Rumble. It doesn't count in judging, to be honest.
Ranger 1 is kind of hard, it took me 3 months to beat Steelix >.> but I crushed the game after that, Ranger 2 is easier, but some things I hate (like when you traded the yellow gem for your friend, I said no 6,000 times (literally) before turning into the dark side (saying yes.)) Trozei is random-ish, it has a short, choppy storyline, it gets randomly hard, and it throws in plenty of boring and unboring levels to get you addicted. Rumble is weak in the way you can only learn two moves, and its outclassed by PMD3.
To be honest, it's hard to decide what the best is...it's probably PMD2.