Never played the first, second was awesome, three was, for the most part mediocre, however the Divine Fortress was amazing. And IV was just all sorts of awesome, and one of the best RPG battle systems I've ever used, it uses this Hexogon shaped Grid, and thankfully the Vth Vangaurd is reusing it.

I love the series because it gives an awesome Old West feel to the game, especially the second one. This is one of my most wnated titles, even more then FF XIII or Makai Wars.