I have been playing with different specs including Grandpa and some NB variants, since hitting 51 (now 56). I have always enjoyed the Assassin play style the best: stealth, move in silently and unleash hell on a single target. I have come up with a spec that REALLY suits me and is a blast to play! Incidentally, it kills faster than Grandpa's too!

Predator: http://rift.magelo.com/en/soultree#wkgw/ykF2Glk928/Tf

I do not use macros. Instead, I have these abilities in line on my main bar. I just go down the line clicking them in order. It is very smooth, puts out insane ST dps, and my survivability is awesome even without Riftstalker.

Expose Weakness
Jagged Strike

Twilight Force
Serpent Strike (can boost it with Poison Malice after TF)
Impale with 5 combo points

Thread of Death
Paralyzing Strike
Twilight Force (keep the buff up) <during the GCD move to the flank or rear>
Final Blow with 5 combo points

If the target is not dead (rare) then hit Savage Strike 1-2x and burn a smaller FB or if Serpent Strike is off cool down burn it.

If you need to immediately attack a 2nd target, replace ToD and PS with Foul Play and Puncture.

L52: pickup Enduring Brew (won't need it that often between poison procs and engine)
L53-55: pickup Improved Final Blow (buffed with Physical Trauma and Advanced State of the Sloth for a total of +45% damage...usually ends the fight)
56: pickup Slip Away

Final 4 points are your choice:
1. You can max Ionization (Tact tree) and Unstable State (NB tree) for an additional +8% non-physical (poison) damage
2. Throw 4 points into Coup De Grace (NB tree) for +8% more damage on Impale and Final Blow
3. Spend the points in the Assassin tree, although I don't see much benefit in this option

This spec offers permanent stealth, 2 CCs (1 ranged), super high ST burst dps, tons of crits, 2 stuns, plenty of self heals, uh oh button (Slip Away), and even a bit of ranged dps via Tact (not high dps but it has its uses).