Riku (SRK) - Honestly, he feels like the a large part of the themes that continue to echo throughout the series: the trials, choices and strength that fortify relationships and friendships. He's a huge reason why I look forward to each game. I just love his journey. He's probably not just my #1 for this trio, but for all of them. Not to mention his design was iconic for me when I was a teen, becoming interested in fashion, lol. He's dark, wears a kilt, has grey hair, etc.
Wakka (TWS) - Kindof arbitrary, considering how little we get to see these three. But Wakka seems like the most fun to hang with.
Aerith (LYA) - Look at me, making up bs trios! Cloud can get bent. No, but really, her first appearance in the series gave me such joy. I'll always appreciate her sage, bohemian presence in any incarnation. (Also, she's a trip in the 4-panel manga strips.)
Donald (MDG) - I love Donald forever, in and out of KH. Goofy is A+++, but I love his wiseass banter with Sora. Gotta keep that hero ego in check, boy!
Pence (HPO) - Smart, quirky guy, into sci-fi goings-on. Not afraid to show his appreciation for The Bouncer. He's got it going on.
Xion (RAX) - The youngest of the organization with yet enough wisdom to remove herself from the equation, against her friends' wishes, in pursuit of the truth. I absolutely love that she elevates Vexen's role and his research with the replica program. Love her themes and boss iterations too, both in Days and Re:Mind.
Terra (TAV) - He's so pure, and I think this quality tends to be overshadowed by how literal Ven's heart of light is. Really, though. He's the hunk with a heart of gold who doesn't judge by appearances, and he's a good "older brother" figure for Ventus. Even if he is more naive than even he is capable of acknowledging. He's neck and neck with Aqua in being misunderstood (she's a literal hair's breadth from being my favorite, because I think she's amazing), which is a great duality given the morally ambiguous tone of dark vs. light BBS focused on.