It depends what type of gamer you are. Xenosaga and Suikoden II, which seem to be the two most popular, are two drastically different games.

From what I've heard, seen, and played, Xenogears is a game geared towards a more philosophical and intelligent player. The story is far, far more in-depth when compared to Suikoden II and there are a lot of subject branches. Philosophy, religion, and politics are quite a big part of the story of Xenogears. If you're into games that will drag you in for the long haul, Xenogears is probably your best bet.

Suikoden II, which I should add is my favorite RPG of all time, however, is nothing like Xenogears. It is a simple story of love, betrayal, and minor politics. The story is easy to follow and has a lot of great twists but they aren't really anything too groundbreaking. The game itself will take you on and adventure and keep your attention throughout, but it sets out to tell a specific story and it accomplishes that marvelously. The fact that it is far less in-depth of a story than Xenogears is probably a major turn-on.

The other two games, while both great in their own right, do not hold a candlestick to either Suikoden II or Xenogears. I highly suggest playing all of the games because they are enjoyable.

Let's, uh, list:

1. Suikoden II
2. Xenogears
3. Suikoden I
4. Chrono Cross