Paladin 10/Cavalier 10, Charging Smite (PHBII) instead of special mount.
Lance - d8
Strength 20 - +5
Magic - +5
Smite - +10
Charging Smite - +20
Unstoppable Charge - x5, x7 on a crit.
adding up we get:
5d8 + 200; on a crit: 7d8 + 280.
For additional cheese, you can get Two-Weapon Pounce (PHBII), which lets you use two weapons at the end of a charge, and then wield two lances.
Sure, your mount will be expendable (you could alleviate that with Wild Cohort, or maybe ride a construct mount or a cohort), but you'll make creatures explode on charges.