Kimori/Juptor/Jukain. He's just so badass, and had one of the more realistic Pokemon developments in the series. It's also not often we see Satoshi with a Pokemon that's power-hungry (rather than his previous Pokemon that had pride for what they were and wanted to prove themselves worthy without evolution, it was evident from day one that Kimori didn't share such ambitions, as seen by how he willingly put himself through a lot in order to score victories against his early opponents).
Chicorita/Bayleaf is my runner-up. She's just so adorable, both in appearance and her affection towards Satoshi. It's a shame she never had many accomplishments past the gym battle with Shijima (I wouldn't count the Shirogane tournament. Though it's nice that she was one of the 6 Pokemon chosen to fight against Shigeru, considering Satoshi filled his team with powerhouses, she got her a** kicked too quickly by Kamex. Likewise for the Hadzuki battle, even if she scored a nice win against Hellger), especially considering she's one of the few starters that evolved (if only once) and was arguably Satoshi's strongest team member through the latter half of Jouto. And I honestly don't mind that she didn't fully evolve, as middle evolutions could certainly use a bit of promoting (they're either cute, or they're powerful, hardly any love for what's in between). I say this a lot, but since the species is proven to learn it in XD, she really needs to learn Frenzy Plant so that she can have something to put her near the level of Satoshi's other two grass starters (and it makes so much since given her proficiency in vines, the Frenzy Plant roots acting somewhat similar).