The Starmantle Cloak is often mentioned as stupidly overpowered, even at the high level when you can afford it.

Also, there are some very fun artifacts out there (though you can't create those yourself as a player, as far as I know). One of my favorites is the Regalia of Neutrality from the Arms & Equipment Guide.

Last and least, Weapons of Legacy (which can be any magic item, not just a weapon). There are quite a few fun things you can do with these, but the costs are almost always not worth it. One thing they can do is open the door for further optimization, since a lot of optimization tricks require the item already have some quality (e.g. Ancestral Relic must be of at least masterwork quality), and turning it into a Legacy item can usually give it that quality (as early as 5th level, you can make your item a +1 improvised weapon, and therefore of masterwork quality). I don't know if that specific example actually works, but you get the idea that Legacy traits can qualify you for other tricks.