Jimmy by a mile. I'm not especially fond of Tracey either ("Come OOON, DAAAD! It's LOS SAAAAAANTOOOS!!!!" and her temper tantrum during "Fame or Shame" made me cringe so hard), and they really didn't do a lot with her storywise, but at least she showed a more likable and pleasant side by being the only one out of the family to contact Michael during the separation. Jimmy I may have found a little funnier, but between his trash-talking and the fact that he drugged his father while they were in the car, kicked him out to the curb and stole his car and took some of his money, yeah, he was a nasty little piece of sh*t. And while he did get a redemption of sorts, by being the one to bring the family back together and showing initiative in turning his life around and actually doing something worthwhile rather than just lay around eating, screaming racial slurs into the mic and jacking off... I dunno, it just felt rushed and not really "earned" to me.
And why was f*ckin' JIMMY the focus character regarding the family aspect? It really should have been Amanda. I guess they were just terrified of exploring more grounded and adult themes like marital strife after the polarized reaction to GTA IV's more mature themes, but it just feels so weird that Michael's wife is sidelined for most of the story in favor of his CowaDoody playing, masturbating, bong-smoking, Mountain Dew gulping and Doritos munching stereotype son. It's an example of what is IMO one of the game's main issues, where you can tell they wanted to explore more mature IV-esque themes, but pulled their punches and fell back on easier, "funnier" situations to try and appease angry fans who wanted San Andreas 2.0.
Buuuuuut I'm getting sidetracked beating on a dead horse. Anyway, both kids were pretty bad (hell, none of the De Santas were saints, and that includes Michael... hell, especially Michael), but where Tracey was just annoying, Jimmy's antics actually left kind of a sour taste in my mouth.