Well, a /Stone Brute is something to think about for your team, but for Mastermind options, here's what occurs to me:
1. /FF is a possibility, and with the Def buffs you have now your entire team will exceed the soft cap. Should make for very safe and fun gameplay. Little will be able to touch you.
2. However, given that you do already have a lot of Def, you might want to think about offense instead. Both /Poison and /Dark are good debuffing sets. The /Poison in particular might allow you to take down very tough targets, and it has a bit of healing and AoE debuff for ordinary fights.
However I think the winner might be /Traps. Debuffs plus the Force Field generator might provide the biggest boost to your team.
3. With a regular play group, everyone taking Leadership is something to think about. Stacking up Maneuvers and Tactics from everybody should be quite a boost. Something to think about it. Taking and slotting Maneuvers for both the Stalker and the MM gives about +10% Def for the whole team. Combined with the Widow and Bane (assuming they slot too) that's about 32% Def for the team -- not counting Mind Link.
4. Primary, doesn't matter. Bots look coolest, imo, but take what ever tickles your fancy.
Someone likes smiley faces...
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what are you igoing to do with this trio? taskforces? ae? mission running?
the brute can be wp if you take pain or ff in your secondary. stone is a very late bloomer and expensive to build around reducing the recharge penalty.
in a group setup with the right buffs, a wp brute can tank as well as a stoner. better even unless he's eating a lot of long bow spec-ops grenades or hefty amounts of defense debuffs while not having any resist shields up from a sonic or therm corruptor.
Because of your particular trio composition, I would HIGHLY recommend a */dark mastermind for the stalker/sneaky bane combo. The stalker friendly dark power is fearsome stare. It's one of the most stalker friendly powers on the red-side. Normally, a stalker/bane sneaks in, whacks a guy, and then his buddies are all over him. With fearsome stare, your stalker and bane can take out two hard/pesky targets or cooperate on one particular devastating foe while avoiding facing their allies. The fear will keep the bandits from grabbing aggro on their alpha strike. With this type of 'pounce' attack plan, the primary group won't matter much; however, I would suggest ranged pets.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
I'd suggest a Bots/Traps. With some IO slotting they can soft-cap their defense solo, much less teamed with two SoAs. It fits well with the whole high-defense theme you have, provides debuffs, and brings a ton of AoE firepower once they hit 32. You've already got hard single targets covered, so a MM than can mow down crowds of minions and bring some debuffs to speed up AV killing should be a good fit. Depending on your friend's budget for IOs, he can also just build as a Tankermind and tank... well, pretty much everything while the rest of you kill things one at a time. 
Thugs/Dark would also be useful... it would give you heals and debuffs plus high-defense minions, but less tanking ability. Or even Thugs/Pain for that matter... lots of healing and some decent tanking, but fewer debuffs. Or just have him go Necro/Dark and keep the entire room debuffed into the middle of next week and cowering in fear as you stroll around assassinating people. 
Cascade, level 50 Blaster (NRG/NRG since before it was cool)
Mechmeister, level 50 Bots / Traps MM
FAR too many non-50 alts to name
[u]Arcs[u]
The Scavenger Hunt: 187076
The Instant Lair Delivery Service: 206636
I would suggest a /Pain Domination MM. Sounds like you have good defense, and decent debuffs.The only thing that's really lacking is some heals,,,,along with the buffs and debuff /Pain has.
I have a Mercs/Pain MM and I love him. The Spec Ops seem to have decent controls,and the damage isn't as crappy as ppl make it out to be. Especially if you slot a couple of Procs in the Mercs.
/Dark is a good choice for the Heal,control,and debuffs. You can never really go wrong with /Dark.
Everyone so far as offered good advice!
A bunch of 50s and too many alts...
Neo's Guide To Thugs/Storm (need to update it)
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Questions about Fire/Rad? Just ask!
Thug for primary since with team and enforcer buffs the pets will be pretty solid. As for secondary /Traps /Pain /Dark in that order.
thank you all for all the replies, im still having trouble deciding however....
primaries to be decided between-thugs/necro/robots in no specific order
secondaries to be decided between-dark/pain/traps/
if someone could please give me an all out attempt to persuade me one direction or the other to a certain powerset it would be greatly appreciated please, im looking for the absolute best compliment to my already awesome trio!
Jordon Justice
No Power will complement your stealth attacking teammates better than Fearsome Stare. The -tohit is very good, but rendering lieutenants, minions, and some bosses moot to your stalker strikes is unequalled. Ranged MM/dark.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
Best thematic fit: Bots/Traps (you're as untouchable as the others)
Best support: Necro/Dark (Dark's controls and debuffs complement single target damage dealers well, and with Necro you have even more controls) or Thugs/Pain (heals are the big thing your team lacks, and Thugs add a ton of damage and work well with /Pain).
Of course if it's just the four of you it doesn't really matter too much... with good slotting and a skilled player, any MM combo on your list can solo 4-man Relentless spawns, and with Bots or Thugs you'll probably clear spawns faster than the other three together can.
I'd suggest Bots/Traps if you want to kill the AVs and Heroes solo too.
Cascade, level 50 Blaster (NRG/NRG since before it was cool)
Mechmeister, level 50 Bots / Traps MM
FAR too many non-50 alts to name
[u]Arcs[u]
The Scavenger Hunt: 187076
The Instant Lair Delivery Service: 206636
Bots/Dark
There is a lot here about why dark works with your team.
Bots add defense which stacks with mind link and leadership.
A game is not supposed to be some kind of... place where people enjoy themselves!
I'd at least consider /Poison too for it's AV killing potential. While /Traps probably does a better job all-around, I don't think you should just write-off /Poison for adding punch to a good team.
That said, StrykerX is perfectly correct. Any well played MM is going to boost your team, so if you have some preference thematically, go with that. I like Bots/Traps in that regard, but whatever seems good to you would be fine.
Bots/traps is perhaps the most versatile thing to play in this game. You have a tool for every situation. Played right, there are very few things that cannot be done. The biggest thing about traps is it tends to require a bit more situational awareness than most sets. You need to know what the situation is before you can apply the right tool.
Perhaps the best tool for your particular team is something to eat the alpha strike. Specifically: seeker drones. There is just something about making the baddies waste thier best attacks on something that explodes in thier faces to debuff and perhaps even stun them.
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I'd at least consider /Poison too for it's AV killing potential. While /Traps probably does a better job all-around, I don't think you should just write-off /Poison for adding punch to a good team.
That said, StrykerX is perfectly correct. Any well played MM is going to boost your team, so if you have some preference thematically, go with that. I like Bots/Traps in that regard, but whatever seems good to you would be fine.
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only play poison if replacing pets with regularity on normal spawns appeals to you
it prob also has pvp appeal, but i could care less about pvp in this particular mmorpg.
[ QUOTE ]
Bots/traps is perhaps the most versatile thing to play in this game. You have a tool for every situation. Played right, there are very few things that cannot be done. The biggest thing about traps is it tends to require a bit more situational awareness than most sets. You need to know what the situation is before you can apply the right tool.
Perhaps the best tool for your particular team is something to eat the alpha strike. Specifically: seeker drones. There is just something about making the baddies waste thier best attacks on something that explodes in thier faces to debuff and perhaps even stun them.
[/ QUOTE ]
seeker drones are our phantom army
Alright guys, so heres the deal, me, my cousin, and my brother all love playing this game together. were bringing a friend who use to play with us, but had to take a break, but hes back!
im looking for the mastermind who would complement our already awesome trio! so if you could give me advice and reasons why you think so and so is the best mastermind combo for us to have please 
i myself play a fortunata, hybrid, good control, high melee damage with all team buffs.
my cousin plays a bane with Extreme melee damage, all team buffs, and all available debuffs.
and my brother plays a dm/ninjitsu stalker, typical insane damage of a stalker, but with two self heals and added defense from our leadership and mind link
so please tell me what you think would best compliment those three characters and if you can provide reasons as to why
p.s.-the only primary we dont like is ninjas, stupid oni! :P
all secondaries are open
thank you to all who reply
Jordon Justice