Depends a bit on just what you mean. If you're creating an item using the legendary/artifact rules from Mythic Adventures, or if you're just looking at an item with a higher than normal total possible enhancement bonus. Different advice for each. I'll try to include some assuming the former.
The biggest piece depends on if you are using a weapon a lot, what level you're going to, and how permissive your GM may be. If you're using a weapon a lot, going to high level, and have any kind of GM, Upgradable is a VERY good investment if you get it early upon a weapon. Effectively getting a +10 equivalent total enhancement for 100k is very sweet. Foe-Biting is also very sweet in this scenario.
If you're GM is a bit more permissive and you don't use a weapon a lot, Upgradable upon a headband or another magic item is equally awesome. It would allow you to start adding other abilities to it perhaps. So creating a ring of deflection AND invisibility for half the normal cost of doing such a thing. Allowing lots of versatility. By a more strict reading though, Upgradable would only allow you to increase the numerics of something. So upgrading a +2 cloak of resistance to +3 or better, but not being able to add say the ability of "Wings of Flying" which you normally could using the additional magic effects rules.
Undectable is always good if you can go invisible regularly. Negate visibility from True Sight and similar spells.
Metamagician is nice for casters as well.
Otherwise the rest are pretty flexible and depends on what your looking to do with your character, and are mostly flavorful choices.