The most underrated game ever. Period.
-Quite possibly the best story that I've come across in any medium. Themes focus on betrayal, revenge, honor, sacrifice, and friendship.
-Art direction and graphics look beautiful, a great improvement over Suikoden 1. The music is also noteworthy, especially the headquarters theme. It's very catchy.
-Combat is a blast in all three modes: duel, party, and turn-based strategy.
-Recruiting all 108 stars is just as fun and challenging as the first game if not moreso.
-A HUGE breadth of characters all unique and memorable, most of which are either playable or serve an important function.
-Luca Blight. The greatest villain. Ever. Hands down. No discussion.
-A ton of mini-games including cook-offs (amazing!), fishing, climbing races, chinchirorin (dice), whack a mole, and many other features that coincide with the story.
-Builds upon the story and concepts introduced in the first Suikoden without confusing new players.
-Develop your own castle and see how it grows and changes according to the stars you recruit.
-Multiple endings add some depth and replayability to the game, if you're playing this game for the first time you owe it to yourself to recruit all 108 stars and see the best one. (Will probably require a walkthrough of some kind since it's very easy to miss certain recruits.)
Bad:
-A few dozen grammatical errors in the text.
-Most of the art and culture is blatantly Japanese (though I tend to like it as is) which westerners may not enjoy as much.
-Only a couple of minor ways to customize the look and feel of your castle.