
Chess Pwn |

Greater magic weapon, And the scabbard give a bonus that doesn't stack with like bonuses. Now if you had a +1 and used greater magic weapon it'd be a +4 for the other class abilities.
scabbard - endow the weapon with an enhancement bonus on attack and damage rolls.
greater magic weapon - gives a weapon an enhancement bonus on attack and damage rolls of +1 per four caster levels
All these classes can increase the enhancement bonus of a weapon, but all cap out at +5 and +10 total.
Magus - These properties are added to any the weapon already has
paladin's divine bond - weapon gains another +1 enhancement bonus, to a maximum of +6 at 20th level. These bonuses can be added to the weapon, stacking with existing weapon bonuses to a maximum of +5
warpriest - This power grants the weapon a +1 enhancement bonus.These bonuses stack with any existing bonuses the weapon might have, to a maximum of +5.
spell warrior skald - [spoiler]grant a +1 enhancement bonus to the weapons (including ammunition) of allies within 60 feet. At 5th level and every 5 levels thereafter, this enhancement bonus increases by 1. The maximum bonus gained is based upon the number of weapons affected: +5 to one weapon, +4 to two weapons, +3 to three weapons, or +2 to four or more weapons. These enhancement bonuses and special abilities overlap with any enhancements or special abilities the weapon already has, though duplicate special abilities do not stack[spoiler]
The sunblade isn't a boost, it's getting set to a new one. So if you had a +5 sunblade, it'd be +4 against undead.

Kayerloth |
So I know it's possible to go above a +5 enhancement bonus with epic weapons and various abilities and spells (Bane and Scabbard of Vigor come to mind), but what's the highest weapon enhancement possible?
Not entirely sure what you are asking, though it appears given responses that perhaps I'm being particularly dense.
+5 is the limit for straight up to hit and damage,
+10 is the limit for total enhancement bonuses.
Both carry the exact same caveat that those numbers are assuming non-Epic or similar play.
With Epic there is no upper limit except what is decided by the GM and the campaign.