My Favourite D20 Add-on is Ken Hood's Grim-n-Gritty system that uses BDB as well as Armour DR Bonuses.
Good BDB: Fighter, Barbarian, Monk, Rogue
Average BDB: Bard, Cleric, Druid, Paladin, and Ranger
Poor BDB: Sorceror and Wizard
The "rule of thumb" is as such:
You choose the best...