Ultimate magic item in a war is...

[drothgery]

What you want here is whatever's the holy relic of the most popular Good faith in your campaign world. Steal it, and bring it to your beseiged city. Scores of clerics and paladins will follow you, in order to get it back.

Presto, instant defense against undead.

 

[JackGiantkiller]

Prayer Bead of Summoning.

 

[Olgar Shiverstone]

With a decanter of endless water I can withstand most any siege.

 

[Christopher Lambert]

The undead could have a bard using a lyre of building. It can build ramps, dig holes (in dirt, through walls, next to rivers... *ahem* ... undead don't need to breathe).

My group did the math behind it, and realized it's ... not balanced. After all, fair is fair.

 

[Captain Howdy]

The Sphere!!!

 

[Elder-Basilisk]

The cheapest item that would make a dramatic difference: Metamagic Rod of Enlarge Spell (any kind really though obviously the more powerful ones will be more effective). Increasing the range of your spellcasters' spells can have a dramatic influence on the battlefield.

If you want an item that would have a really dramatic effect, I'd go for a censer of summoning air elementals. Once Air Elementals get to huge (or even large really) they can vaccuum up hordes of enemy troops since all they need to do to kill an ordinary foe is fly over it in whirlwind form.

 

[Felix]

I'd go with a Holy Heavy Catapult of Disruption +1.

 

[Angcuru]

The sun, some mirrors, and a really big magnifying glass. OF DOOM!

 

[cignus_pfaccari]

The arc.

*cues Indiana Jones music*

Actually, in general, the Ark would be the most useful potential item in war, as one of its powers is to make the army that follows it into battle invincible. (GURPS Warehouse 23 states that the Army of Virginia's battle flag had a representation of the Ark, and only really started losing when they replaced it with another...)

On a more personal level, the Spear of Destiny would be good as well, as it has the same broad connotations, but that's more of a "climb to power" kind of thing.

Brad

 

[RangerWickett]

The Holy Hand Grenade of Antioch.