CohortsFiends of Possession make optimal cohorts for characters of the appropriate alignment. The key feature is their ability to augment your character and remain out of sight. One of the biggest problems I have with leadership is picking a cohort that is useful, out of the way, and does not overshadow other party members.
The FixerThis cohort can make any armor, ring, weapon, or wonderous item, plus buy and sell things at the best prices. You can leave him at home when you want or take him on the go. The xp mechanic with leadership makes the cohort useful even after he hits his max level, just continue to burn the xp or hide it with the link below.
Midguard Dwarf 8 HD +4LA/ FoP 5
L1 Mercantile (if available)
L3 Item Familiar (if available)
L6 Master Artisian or eberron crafting feat
L9 Master Artisian or eberron crafting feat
L12 Master Artisian or eberron crafting feat
L15 Open
Sanctify relic might be of interest if your main character is a divine caster.
http://www.wizards.com/default.asp?x=dnd/we/20060526aThe standard FOP build in the two sections above is the other optimal cohort build.