plant/emp controller, you'll have something for everything but slotting/powerchoices are a bit miffy. Got mine to level 23 atm and its GREAT for teamplay.
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I would say plant/ta or plant/rad
Plant/storm, the AoE damage is just ridiculous. Mine only 24 but already hot to trot.
Failing that, a Kin defender would also be great with the other 2 toons.
Storm/Psi
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Storm/Arch/Dark
Seriously though if you want a support toon that is useful in ALL taskforces roll a cold defender.
If you will spend most of your time as trio i suggest you a blaster. Blaster's are always useful. Maybe something AoE heavy like Fire. With a tank and illusion trollers pet you will barely draw aggro. My personal favorite for secondary is Electric.
Scrapper is another good choice also as Rad's only heal is PAoE, your rad friend can heal you both at the same time. For set, don't pick something do solely smashing or lethal damage, Fire or Dark is a good choice maybe Spines but don't know its Toxic damage percentage. I am saying this depending on only one TF, Lady Grey TF. Last boss immune to S/L damage as far as i heard. Not resistant, i say immune that means he don't get any damage from that type of attacks.
If i was the one to join that two i would roll a blaster.
i got a cold defender, emp isnt really my style, i prefer to kick out some dmg, but plant is definatly seeming to stand out abit, i have earth/ta so maybe a plant/rad would be nice! Thank for all the reply's
Storm / Sonic Def ftw. xD
Huge -Def Huge -Res,
Dmg + Pet + debuffs,
Aim + Power Buildup.
Great support, great Damage, and respect from everyone
when you learn how to use Hurricane in a team.
xD
Consciousness: that annoying time between naps.
lol hurricane isnt that activated for critter football when u have a corridoor in your sights 
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i need a toon that will be useful for Every TF, bare in mind ill be teamed with an ill/rad troller and an invun/ss tank. Personally i was thinking a mind/... plant/... troller or BS/... DB/... scrapper or Psi/... sonic/... but would appreciate any suggestions to mix with the above or anything else u have in mind, MUST be useful for ALL Task Forces!