for me it's grand prix, epic astro, then the soccer one.
the soccer one is fun but the new game+ options retain the stats from all the old players so it becomes too easy. there's still challenge in grand prix, probably the most balanced. epic astro is good 'cause it's kinda original compared to their other games.
i found the weakest one to be hot springs, but all of them have that addictive quality that you can't put down... and it lasts for only a week but still.