You need several things in a team. I'm not sure if two people can bring all of them, but you can come close.
You need someone to control who gets attacked. Usually this is done by a Tanker, but a Scrapper or Brute can do this too. Some Controller sets offer options to prevent most damage to teammates.
You need the person being attacked to stay alive. Either you use defense buffs, healing, or that person has enough self-healing to stay alive. Again, a Controller may have options for you.
You will want DPS. If you don't kill fast, then its not an effective duo. AoE DPS would be preferred over single-target until you start facing AVs.
You may want debuffs. Defense debuffs make it easy to hit things at lower levels. Resistance debuffs make it easy to do more damage.
You may want offensive buffs. Kinetics is the best offensive buffer in the game, but other sets offer offensive buffs.
Figuring out the exact best combo is hard. There are too many options to choose from. Usually, you will lack something with just two people. You may also lack the ability to focus your characters if you try to spread them too thin to perform too many roles.
The problem with "the best combo" is there are so many of them. Really, you should play what tickles your fancy. It'll probably be just fine, and you'll have fun. Don't think that any one power set is required to play. All of the sets are good and you can play any of them with success.
That said, some ideas do occur to me:
Villain Side:
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2 Masterminds, of any sort.
1 Mastermind, plus one of anything else.
1 Thugs/Traps mastermind, 1 Thugs/Thermal mastermind.
1 Dark/Invul Brute, 1 Fire/Rad Corruptor
2 Veats, of any sort
2 Crab Spiders in particular would be strong.
Hero Side:
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1 Invul/Dark Tanker, 1 Fire/Empathy Controller
1 Fire/Shield Scrapper, 1 Dark/Dark Defender
There are a LOT of good combos. Here are some that my wife and I have had great fun with:
Fire/Ice tank and Fire/Elec blaster
Rad/Sonic defender and MA/SR scrapper
Two empaths with special builds
Mini-guides: Force Field Defenders, Blasters, Market Self-Defense, Frankenslotting.
So you think you're a hero, huh.
@Boltcutter in game.
The generic answer is 2 Emp/Sonic Defenders.
I would go for more variety personally. Fire/Stone tanker with Sonic/Cold corruptor. Elec/Dark scrapper with Earth/Kin controller. Fire/Fire blaster with Bots/Traps mastermind. Whatever gets your gun off.
They ALL float down here. When you're down here with us, you'll float too!
@Starflier
My wife and I have played logs of duo combos
And
When she is not around there are several I can play controlling both using our side by side computers
Best is two emp/ sonic def
It eventually got boring
Never died once until lvl 34
More fun is any scrapper or brute with a def of your choice
Our fav is kin/sheild brute with emp/sonic def
My next combo will be a pair of fire/rads for AV melting
Or Ill/ rad troller with fire/Kin
For balanced large group and av killing
Laserblitz
"Kicking digital a$$ and taking "trademarked by NCsoft" names since 2007"
If you see a rainbow costume or named "Emcc" it is me
My favorite duo is a Fire/Shield Scrapper and a Fire/Cold Corruptor. They do great damage and Cold as some great debuffs. Cold gives the Scrapper +defense which will put them close if not over the soft cap. It is a great duo and a lot of fun.
"All right, they're on our left, they're on our right, they're in front of us, they're behind us...they can't get away this time"- Chesty Puller US Marine Corps
Best duo? The best duo is two people that play together a lot. Mrs. A and I duo a lot, and have been for almost six years in this game. We can duo almost any combo pretty damn well, because we've played together a long time and know what each other will do in almost any given situation.
A pretty good metaphor for our marriage, really, although that's been nineteen years rather than six.
I used to fiddle with my back feet music for a black onyx. My entire room absorbed every echo. The music was . . . thud like. The music was . . . thud like. I usually played such things as rough-neck and thug. Opaque melodies that would bug most people. Music from the other side of the fence.
Many offerings have been given and many more could be. Few years back I really liked a duo I had with a freind: She was Rad/Sonic Def and I was Kin/Dark Def. You got debuffs, buffs, -damage res, -acc. When we joined teams things were so smooth as well.
My suggestion is to go with different AT's. If you go with same AT's, choose different powersets.
This has nothing to do with effectiveness, but more to do with not feeling samey-same. That gets boring. This suggestion is to maximize fun since everything will work just fine when it comes to nuking NPCs.
Well lets see me and my wife have several.
Ele/Sd brute and a Ice/therm troller been steam rolling mobs with these two.
EA/EM tank and a EC/Rad troller that are in their 30's we have been doin tf's and duo'in GM's.
KM/WP and MA/WP scrappers just fun all around.
Me and a buddy ran a fire/kin and a elec/ice trollers and steam rolled Winter Lord and Adamastor for fun and they worked pretty well together.
I believe one melee toon tank,scrapper,brute combined with a a good support toon always seems to work great.But I'm also seeing acouple support toons together can be pretty deadly also.
I don't think there's any good answer. Even unlikely sounding combos can work well depending on the builds involved and the players behind them. My rad/rad corr did a pretty effect duo with a defender the other day, for instance. I would say experiment with a couple of things until you find something that works or you.
My comrade in arms and I plan on making several characters to duo all the way to 50 together. We're not going to be doing much teaming with anyone else, just the same two all the way. So what are the best/most efficient archetypes/builds to duo with?