Lucario can utilize Calm Mind, Bulk Up, and Agility for massive buffing. However, that only leaves one moveslot for an atack, and he can't learn Baton pass to pass all of these buffs.
The next best choice would be Girafairig, which has Baton Pass, along with access to Calm Mind, Agility, and Double Team.
A versatile choice is Illumise, which can use Flatter and Swagger, followed by a Psych Up, then Baton Pass.
Another good alternative is Floatzel, which can get Baton Pass, along with Agility, double Team, and Bulk Up.
Sorry about adding the Baton Pass idea to this question. I'm assuming you want a good baton passer. Even still, these are the best choices.
EDIT: There are many honorable mentions, such as Pikachu (Although it has no access to Baton Pass.), Blissey, Plusle and Minun, and many others. You'd be surprised at how many Pokemon can buff themselves to incredible levels!