spines/ninja if you do like EM
MA/Nin for PvP...
Ninjitsu will provide a good heal, placates etc.. good defence. And for Warburg you have +perc for Stalkers.
Mine is Ninja/Regen, but I'm thinking of remaking him to a Ninja/Ninjitsu. Regen Stalker just doesn't do it for me.
Other then EM MA is the best since it has the highest damage single person attacks (any attack that can hit 2+ foes does not auto crit from hide).
Nin is a good set but you need to rely on BFs or Acrobatics vs KB which can be a strong weakness.
Regens tend to get integration detoggled alot and as such left vulnarable.
SR is probably the best as, esp with elude, it helps most in keeping hide on (through avoiding damage) and has +perc.
That being said Energy for its res to TP foe may save your skin often aswell.
As long as you never stick about though secondary doesnt realy matter too much 
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Im considering rolling a stalker, mainly because im nored of a brute, and i messed up my build, unsure of when i can try and respec.
What id like to ask, is what would be a good set for a Stalker who would like to PvP? im thinking MA/Something, mainly because im not too keen on glowing pom poms.
Thanks in advance.