I have a small improvement on CyberSpyder's Diplomatic smackdown.
With the Book of Exalted Deeds, I can get higher than his +53 -- even without the custom +10 ring of Diplomacy.
The character is a half-elf Clr1/Rog14 (or Clr1/Rog13/whatever1). He/She must be good, beholden to a celestial entity, and vow to abstain from harming any living creature. The only stat required is base Cha 17; even low-Int characters have enough skill points for this path.
This requires 8 feats, the base 6 plus two from rogue special abilities: Cosmopolitan (Diplomacy), Courteous Magocracy, Negotiator, Nymph's Kiss, Sacred Vow, Skill Focus (Diplomacy), Vow of Nonviolence, Vow of Peace
19 ranks of Diplomacy
+6 synergy
+11 Charisma
+3 competence (circlet of pursuasion)
+4 sacred (Herald or Joy domain)
+1 luck (stone of good luck)
+2 racial
+2 perfection (Sacred Vow)
+4 exalted (Vow of Peace)
+2 Negotiator
+3 Skill Focus
+2 Nymph's Kiss
+2 Cosmopolitan
+2 Courteous Magocracy
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Total: +63