Stun Monster (Energy Melee/Dark Armor) AT Decision
For an actual stun monster, you'd actually get more mileage out of DA/Stone. Because EM only has chances to stun (and pretty low chances at that), you're not going to be perma-stunning everything as much as you'd want. OG + Fault will keep everything except for AVs permanently stunned without using nearly as much animation time devoted to it, not to mention that Stone still has excellent ST damage and substantially better AoE damage than EM.
Of course, this precludes you from going with a Stalker, so you'd probably just want to pick based on which side you want because both are going to be decent choices (the Brute is gonna have a hard time building Fury though).
I too would suggest considering DA/Stone instead of DA/EM for a stun-beast unless you just wanted to stack stuns on single targets. DA/Stone would likely fit better as a tank than a brute because of the effect the perma-mag-4 stun will have on fury generation.
If your concept demands Energy Melee though (dark little beastie w/brightly glowing pom-poms always did amuse me)... I'm not sure which AT. I would generally say to avoid /DA stalkers because stalkers tend to work better with defensive sets than resistance sets, but you said you were going to IO for max S/L defense anyway, so that would be pretty impressive. I would still likely say to avoid the Stalker, since you would have zero AoE output; the brute would usually offer more damage on a team setting with whirling hands and death shroud available. DA Tanks can be scarily durable, esp. with a buffing defender (ESPECIALLY w/a Cold def) to back you up. Anyway, if you're going to IO for defense to layer on top of your stuns, Dark Regen, and DA's considerable resistances, the durability of a tank may be excessive for your needs, which leads me to say Brute. Anyway, go with your priorities: if you demand large damage output, brute it up, if you don't care about the damage as much and want an impossibly hardy stunning machine, get a tank.
Rule number six of an empathy defender is NEVER underestimate a blaster's ability to die. I don't care if he has CM, Fort, both RAs, bubbles (both FF and Sonic), and is fighting next to a Storm defender with hurricane on. If there is a way to die in that situation, the blaster will find it.
DA/Stone will be almost prohibitively end heavy.
@Mechaniker
Official Old Angry German Guy of CoV.
My Characters: http://boards.cityofheroes.com/showthread.php?t=247787
That and Stone Melee is ugly and doesnt fit the concept.
Thanks for the input guys! I believe I'll be able to settle on a decision now.

@Mazzo Grave

Webmaster Grave, Virtueverse!
Energy/Energy Blaster Guide
Theme-wise, this character coud easily fit as any of the melee ATs, my only problem is, which would be a better choice? A stalker for the stealth and slight defense buff (I'm slotting the build primarily or Smash/Lethal defense), a Brute for pure power, or a tank for increased survivabilty? The combo was selected specifically for stacking stuns (Energy Melee + Oppressive Gloom), and with that in mind, which AT do you suggest?
EDIT: Found that I can get more Defense out of a Stalker, more overall DPS as a Brute (thanks to Fury and the damage aura/whirling Hands) and more Resistance and HP out of a Tank, with less defense than a stalker, but more than the brute version. I'm not sure which would make for a better Decision still. :/
@Mazzo Grave
Webmaster Grave, Virtueverse!
Energy/Energy Blaster Guide