I have them all accept for Tierkreis, (I don't have a ds) And I really enjoyed them all. It's hard to pick one because they all have things I love and things I don't like. I voted for Suikoden 4, it had a great story, and I loved seeing Ted again from suikoden 1. They're all really close though.
Suikoden 1: This was the first suikoden game I ever played and the 2nd or 3rd RPG I ever played as a kid. I have very little complaints about this game, it had great music, great story, it was loads of fun.
suikoden 2: Again, a great story with probably the greatest RPG villain of all time. I didn't much care for the monster characters, and I was a bit pissed when I found out I had to kill Neclord again, but other than that, awesome game.
Suikoden 3: At first, this game didn't really do it for me because I had already played FF10 and I was expecting voice acting. A couple of years later, after having already beat Suikoden 4, I decided to give this game another try, and I absolutely loved it. My only real gripe was the lack of voice acting andthat the war battles were irritating.
Suikoden 4: Loved the story and how it was very Pirate themed. I just hated navigating the world map, thank god for the blinking mirror.
Suikoden Tactics: More of a spin off of four, than a new game. This game was probably the weakest in the franchise. The fact that it only had 64 characters was a bit of an insult to Suikoden games. Also, I found it obvious who the villain would be as soon as you met him. The story and leading characters were all pretty lame as well.
Suikoden 5: This one was probably my least favorite. I've only finished it once, and it's the only game where I didn't manage to recruit all 108 stars. That was one of the bigger problems for me. I couldn't even figure out how to recruit any of the merchants without a guide of some sort. I'll probably give it a second go sometime, but until I do. This one sits at the bottom for me.