Coming from someone who's beaten Agarest 1, Zero and is currently playing through 2:
Agarest 1 is a great game, with an absolutely amazing battle system and amount of customization. The game is very slow, and is all about strategy. So don't expect any fast paced action or anything like that. If you enjoy strategy and creating characters the way you like them, then this game is for you. The fan-service is also very mild, despite the high emphasis on its marketing.
Agarest 0 is a good game. For better or worse, it's essentially the first game with a new coat of paint and some extras thrown in. The characters are different... but their personalities all almost directly mimic those of characters in Agarest War 1. You can pretty much match up their counterparts. The sound is higher quality, and they've added a bit of animation to the character portraits during dialogue. The main other change is the inclusion of "vacation days", which are a pretty bland way of giving you character interaction and chances of getting bonus items.
Agarest 2, from what I've played, is vastly different from its predecessors. The combat overly complicated and loses much of the strategic elements that came from the originals' simplicity and polish. It manages to be faster paced and more interactive but also feels unpolished, clunky and too complicated. There is more animation this time around and the sprites are of a higher resolution. However, they are mostly animated by tweening, which seems cheap and lazy. The game also has pretty poor pacing. It throws a heap of battles at you, until your begging for dialogue and events, then it drowns you with those until you're craving some battle to mix it up. As of my current experiences, it has not found a good balance between the two. The game has also been obnoxiously easy up to my current progress. It's nearly impossible to lose any battles. I hope this game balances itself out as I get further in, because as my current experiences with it, the first two are much better.