Depends on the weapon, your build, your role, and the level of content you are doing.

Yeah, that's real helpful, isn't it? Here's some general rules of thumb that should explain what I mean.

• Sharpened if you need penetration, but not if you are doing content where you can expect your group to fully or over penetrate the boss' resistances, such as with a regular trial group.
• Precise if you need more critical chance for your build or to proc a set bonus, but most DPS builds are in the 50-70% crit range for regular criticals anyway. Given the preponderance of impenetrable armor in PVP, I wouldn't recommend precise unless you are doing a full crit build.
• Infused is useful if enchantments make up an important part of your build, or you are using a set like Torug's Pact that works with enchantments. Infused is a good choice if you know you don't need the penetration from sharpened.

If you want to get more specific, you need to think about how it fits into your build and the role you want to play.