Defining "killer combo" to whatever is convenient to your argument is just being argumentative.
A killer combo is a combination of cards that results in an amplifying effect that makes the group more capable as a whole than as individuals in defeating enemies. I base that on the first post, which was intended to set the stage for the discussion. He talks about "hot damage", so his intent was combinations of cards from different characters that amplify damage more than normal. I will expand that to being "more effective against the enemy" than just large damage because some enemies are not just about dealing damage... but more effective combinations in winning against a broad range of opponents.
That said, each villain is different. Some are disadvantaged by area damage, and some by direct damage. There are specific combinations that are effective against specific enemies. The best team to take against an individual villain is not the intent of the originator of the thread. So the combo must be effective against multiple enemies.
Legacy + anyone does more damage, so his very presence is a combo. For a Legacy combo to be more "killer", it needs to be with the Heroes that use it to greater advantage.
For instance, Galvanize + Squall does baseline 2xtargets in damage. That seems like a killer combo against some villains, but not all. It is very effective against Infinitor's weak Manifestations, but bounces off Akash'bhuta when she's got lots of arms out or Apostate's Relic damage reduction. It is also contraindicated against Voss (because it tends to flip him) and Citizen Dawn (losing control of the Citizens in discard due to it being indiscriminate), but especially Dreamer since it kill sher and you lose. It is also no help against villains with very low villain target counts in the deck. So I can't call it killer. It is merely situationally more effective.
This differs from the standard AZ+Legacy combo. With his modules in play and Legacy giving +2, Thermal Shockwave heals AZ for 1+2x2 while dealing 2x(1+2) damage. It can instead deal AZ -1-2+6+2=5 damage, deal one target 1+2, and a second target (1+2)+(8+2) damage. Legacy's effects are dealt 4x to one target at the cost of 2x to AZ, which puts Fanatic to shame for her version of "hurt self to hurt others". It is flexible and can get even worse with a third target instead of healing. It even penetrates higher Damage reduction, which someone earlier commented about with reference to Fanatic's effect with Legacy.
Since I only just got WotC Thurs, I hadn't had a chance to experiment with the decks yet. CC and SS work extremely well in combination, but are not a killer combo. They cover each other's weaknesses extremely well, and raise reliability of victory vs. other pairings, but do not amplify each other. They fill each others' holes. SS, for instance, taking all damage from hero targets protects the constructs. And CC gives SS a way to do direct single target damage. There is a synergy there, but not a killer combo. SS seems a shoe-in as best combo with Unity. She also gets Legacy's Ring into play, which has always been Legacy's weakness. One of the best five card teams may wind up being Wraith+Legacy+Sky-scraper+Absolute Zero+Unity (in that order). Ongoing and environment destruction, lots of equipment to sacrifice, healing, Villain Deck control, villain damage reduction and control, and the raw damage AZ can flood out.
CC does have an effective combo that might be killer, but it is a healing combo, rather than damage. Vitality Conduit+Motiviational Charge + any ability that hits a Hero target for 1 damage (like Thermal Shockwave). Shockwave hits VC for 1, heals target for 2, later MC hits something for 2, heals everyone for 1 including the VC. Rinse and repeat. Further enhanced by Dynamic Syphon, which allows this to go off every Hero Turn for each Hero that can damage DS and VC. Killer? Since it works against almost every enemy, I'd say yes. But Legacy can't buff up damage anymore, or the Constructs die, so it is a post-near death recovery combo. Cosmic Crest helps with another DS on CC, but now we're getting into stupid numbers of cards. In our case, we had Tempest out, too, with Healing Rain, but I thought that was too much healing and was lengthening the game unnecessarily.
Nightmist + Tachyon + Visionary can combine well. They feed cards into each others' hands, and two make good use of them. (I look forward to Super Scientific Tachyon with Nightmist online with Relics, but quite a wait for that.) Much as I like Nightmist for her fun factor, she comes out soft far too often, and can tear herself up trying to become effective, then wind up unable to get herself healed back up fast enough to participate. Early on, this team has weak damage output and no healing except personal, so can wind up letting the enemy flood out. The weakest point is Visionary, who seems to die first every time.