I can honestly say that my Thugs/Dark is my favourite mastermind. I always enjoy playing her and once you get your Bruiser, you can literally begin steamrollering your way through missions. I heartily recommend.
Down side is that you'll need most of the powers from /Dark to really rock hard and the playstyle is very active. If you enjoy passives, go FF but that bored the crud out of me after a while.
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Would have to say Dark, but then I'd pick that as a secondary for all but one primary as my first choice.
Yes its very active, but hell as an MM a passive secondary would drive me nuts
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
Dark is certainly the strongest secondary for MMs. The Heal and Res alone make it worth taking, and there are no fillers in the dark set, they are all good.
Having said that, if you are getting bored of Dark (I have it on 3 MMs and that's enough really!) Trick Arrow is surprisingly fun, and keeps you busy.
Like most i really like Dark as the Secondery. Great for solo play and team play. Not a PvP'er but i can't see it being a bad choice there either.
I'm playing a Bots MM just now with Traps and it's ok come the 20's but i still prefer Dark overall.
/Poison is a much underestimated set. It has got some of the most powerful single target debuffs in the game. Thugs in fact is the best primary for poison too, the bruiser is perfect for the tier9 poision power. If you want something a little different I would try this. Its AoE crowd control isn't brilliant, but then I've always found that that my MMs have less trouble with groups and more trouble with hard hitting enemies, the kind of enemies /poison excels at. Its not an easy ride though, but poison has the potential to kick bum.
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I'm saving Poison for Ninja's cause I dont find Dark works with Ninja's screwy AI too good
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
Thugs/FF is so overpowered that I ended up deleteing mine at lvl 20 something out of pure boredom. You can get rid full team spawns without breaking a sweat.
Forcefield on Mastermind is mind-numbingly boring, even more so once you get PFF. You're safe as houses most of the time. Poison has really nice stuff in it, as JawD said a moment ago. And Storm might be a bit chaotic with thugs. So pretty much FF for "safe mode", Poison for Control effects and Storm for simple chaos I guess.
And Dark if you want to be like everyone else and actually be quite awesome 
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In theory, you can choose any powerset and not choose any of its powers, for thugs I chose pool powers as a secondary that help me in battle such as fighting, tactics and Presence, these all fit into what I call a 'Natural' Thug mastermind
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I only have the one MM and it's only at lvl 22 but Thugs/Poison is teh powerset I chose and I have yet to regret it debuff the big sod and it's easily taken down in seconds.
No love for Traps in this thread 
Thugs/Traps is astoundingly good fun with pets and minions everywhere (Acid Mortar can pull for you, seeker drones are one of the best alpha absorbers in the game). And Poison Gas trap is still decent post-change.
I think I actually prefer Traps to Dark for a MM, it seems more Mastermind-ish (altrhough Dark is my fav Defender primary).
I've run a thugs/ff mastermind up to mid 30s.
All the defence buffs available in the two sets seem to make the thugs fairly invulnerable in pve.
On large teams I seem to be constantly busy, actively controlling the minions whilst trying to keep minions and team-mates bubbled. It's not a particularly exciting busy though, and direct intervention with the good guys is limited to force bolt.
I've overplayed Dark on thugs way too much. I'm currently working on a Poison Thugs MM, and it's working out surprisingly well. The debuffs are quite something with the thugs overall strength.
And a few of us are talking about Thugs/FF? Sounds...awesome.
Hey guys okey so im gonne make a new mm and i decided on thugs but i cant decide on a secondery i was thinking of storm/poisen/FF . . . . Any advice?