Obvious one, it's Altairs armor
SPOILER
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Gained from gathering all 6 plates from the legendary assassin's tombs, then delivering them to their graves beneath your mansion in Monteriggioni.
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/SPOILER.
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In Assassin's Creed 1, all the weapons are pretty balanced, so we can leave that out.
In Assassin's Creed 2, the strongest weapon would be the Sword of Altaïr, but the deadliest weapon would be the double hidden blades.
In AC Brotherhood, the Dagger of Brutus appears to be the best weapon to my knowledge.
Both the Master Assassin's armour and Ishak Pasha's armour are 5/5s on every scale (deflect, health, etc.), but Ishak Pasha's armour sports additional resistance from firearms. It is the strongest, and can only be acquired by collected all 10 of his memoir pages, scattered throughout Constantinople.
In GameSpot preorder trailer we hear that sword at bonus mission "Vlad the Impaler's blade" is, and I quote "deadliest weapon Ezio has ever wielded". Looks pretty awesome too.