This is an old build of mine, but I figured I should start a thread for it. The only book required is Complete Warrior.
X 5/Purple Dragon Knight 2/War Chanter 10/X +3
X can be any full-BAB class, though a White Raven focused crusader probably has the best synergy. The cohort has 10 levels of kensai, and the rest of his build doesn't matter.EDIT:
A faster entry is Paladin of freedom 4/War chanter 10
with the Harmonious Knight
ACF. Paladin of slaughter works too.
The two class features you're looking at are Instill (kensai) and Inspire Legion (war chanter). The former lets you transfer some of your BAB to another character. The latter lets everyone around you use the highest BAB of any character in the area.
1. Have your cohort Instill 10 points of his BAB into you, raising your BAB to +30.
2. Use Inspire Legion, setting everyones' BAB to +30 (including the cohort).
You can take this trick further by giving the cohort bloodlines, levels in uncanny trickster, etc. to raise his effective kensai level and thus how much he can instill. Or just use multiple kensai.