Heironeous
The standard Paladin god. Greyhawk's Lawful Good god of Justice and Valor, Heironeous the Arch-Paladin smites ALL the evil, especially anything related to his hated brother Hextor. His symbol is a sliver lightning bolt, and his favored weapon is the longsword. This is a recent and deliberate change, actually; he's known for his battleaxe, but is trying to broaden his appeal. Your cleric can choose to use either.
The world is a dangerous place that poses a never-ending series of challenges to those who fight for justice and protection of the weak and innocent. One must act honorably at all times, and uphold the virtues of justice and chivalry in word and deed. Danger must be faced with certainty and calm, and glory is the reward for defeating evil, while virtue is the reward for upholding the tenets of Heironeous.
His domains are, obviously, War, Good and Law.