Primary isn't that relevant for the activity, for secondary, dark is most active I'd say, followed by poison and TA.
Bots are heavy AoE, but you'll have to wait until the level 32 upgrade. The same goes for Thugs and Mercs. MMs aren't AoE masters until level 32.
As for activity, it depends on a few more things than powersets. You have to consider your own playstyle, how you specced your character and what you consider activity. Are you constantly giving orders to your bots? Are you using your powers when they are ready and needed? Have you picked powers with long or short activation and recharge times?
So no sets that prefer melee has good AE? Ninja? I hear the tier 3 pets has AE damage.
By active I mean, debuffing, healing etc all the time. I
@Boerewors
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So no sets that prefer melee has good AE? Ninja? I hear the tier 3 pets has AE damage
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I hear that necro and ninja have good melee pets but Im not sure.
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So no sets that prefer melee has good AE? Ninja? I hear the tier 3 pets has AE damage.
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For the ranged all their hench get some sort of damage AoE (except the Spec Ops) with either the first or second upgrade. With the melee hench, only the Oni gets AoE damage and then it's limited to Fire Breath and Rain of Fire, RoF takes skill to use effectively (ie. not screw up things). The hench aren't all that skilled.
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By active I mean, debuffing, healing etc all the time. I
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I'd say that Poison may fit you better than Dark, even though Dark is numerically better against large groups.
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I have a lvl 20 bots/dark and its (yawn) not very active at all.
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Not at level 20 it isn't. After that point, it's as active as you care to make it.
@Synaesthetix
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Union: FU//LoUD
"that Syn is that that" - Mothers Love
If you want a working MM, try /traps. They work harder than most simply because of the entrapment, patience and forward thinking approach needed 
Solo, you must be patient, there's a degree of AoE here depending on your point of view
Teamed, slightly harder but given a patient team or the ability to work under fire and you'll still shine.
If you take Mercs or Thugs as your primary, you can still play an active roll too.
"Like War_Hero said, leave it as it is, it toughens up the softies." - Naz Nomad
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My robo/poison MM is very active and when I am in the mood I switch my bots to bodyguard mode and even use Sands of Mu and Air Superiority on top of debuffing stuff. (Snaptooth didn´t like that one bit. 
)
If it has eyes, you can blind it, if it has blood, you can make it bleed, if it has a mouth, you can make it scream.
I am currently playing a necro/dark MM and having a whale of a time as it is pretty active and has lots of control options but sacrifices for damage, most particularly AE damage.
I love the MM playstyle and an active one as well, can anyone suggest a MM set that is active and doesnt use dark? I have a lvl 20 bots/dark and its (yawn) not very active at all.
Also looking for something with realy good AE damage
@Boerewors