Here are a few of my most flexible and most powerful assassin builds.
Note: Some of these builds maybe look similar to builds you have created or have seen.
Purpose: Pve, Pve
Difficulty: Hard
Reason: Many times in Pve the elementalist is the reason a group fails and this build will allow you to take out one of them with out it whiping your group in the process. In Pvp this is very effective against monks due to the fact that if there is no other monk there are only about 2-3 skill that can stop thing from killing them (ie. Signet fo Devotion, Holy Viel, Hex breaker) in which case you will have to rely on your tactics rather than the build.

The Skills:
Siphon Speed: For X Seconds target foe moves 33% slower and you move 33% faster.
Shroud of Scilence: For X Seconds Target Foe cannot cast spells and all of your spells are disabled for 15 seconds
Black Spider Strike:Offhand. Must Strike a hexed foe. Target is struck for +X damage and is poisoned for X seconds
Twisted Fangs: Dual attack. Target for is struck for +X damage and suffers from bleeding and poison for X seconds
Impale:Target foe is struck for X earth damage and suffers from a deep wound for X seconds
Signet of Toxic Shock: If target for is suffering from poison, target for takes X damage.
(Optional Healing/Condition Removal)
Resurrection Signet

Attributes:
Dagger Mastery: 9
Deadly Arts: 15**
Shadow Arts: 6
Critical Strikes: 9

Usage:
At first this combo may look very weak but it actually does lots of quick spike damage and Signet of toxic shock is unarmored damage.
Although Shroud of Science is very powerful and devastating to most build you want to try to avoid using it as much as possible. It is to take out KEY targets and not simply an assassin even though it would take away the most common healing they have. Shroud should only be used on the following:

Monks
Hexes after finding out what they are (Elite wise)
Elementalist that are either pressure or blinding.
The last person standing

The biggest pro about this build is that there are only two attack skills in which most people that try to blind you do not even have enough time to react to it before you switch to you impale shock combo portion. You will only need to chase for a short time before they are dead. Even with warriors Impale will do about 80 damage and Sag of toxic shock will do its full damage.
The Optional healing/condition removal is based on where you are using this. In the Team arenas when you have a monk it is better to take mending touch than anything else (feigned Neutrality is also an option). In the random arenas Shadow refuge is the best choice since Feigned Neutrality doesn't truly HEAL you; it simply keeps you alive for its duration. If you are in Pve Heart of shadow is the best choice unless the area calls for something else.

Some Helpful Hints:
For PvP:

You are an assassin, not the first one to go in. Let the battles start (unless you are in Ha, Gvg, or Ta) and pick of the most important targets FIRST but only after they have picked their targets. Also the mind set of "first one in, kill on sight, if you hear someone call you a noob, Destory on thought." does not always work. always think about what will happen if you do a certain thing.

For Pve:

*sigh* You know this, You are not a Tank, You are not to go in first, You are not a Tank, Pick of the important ones, You are not a Tank, Kill elementalist first, YOU ARE NOT A TANK!

These are not all of my builds, I have at least 15 more that I could share but these are the most balanced ones (not the strongest)

Consepts:

Not being a tank
Try not to be the main target
Twisted Mark Combo
Impale Shock Combo
Healing is based on Location
Stay on task (Just Business, Nothin personal)
Snares (All of the builds that are for PvP usage had one)