Thanoseid vs the JLA or Avengers
Take the best of the two of them, Thanos and Darkseid.
Give them all their toys, IG, Omega Beams, etc...
then combine them into ONE being with the combined abilities, strength, etc.
THEN have them run a universe domination plot against
1. JLA
2. Avengers
3. Justice Society
4. X-men
how does it end each time?
What about all the teams at once?
The way I see Thanos or Darkseid is that they are threats to the teams mainly because they are smart, they plot and plan their moves and are ahead of their enemies each and every time, even if they loose they gain something out of their lost or the lost was planed all along and was part of a bigger scheme.
Unlike Luthor or Doom who have only the intelligence to achieve their goals, Darkseid and Thanos ALSO Have the physical stats of TOP TIER characters, which in combination to their intelligence makes them very real threats to any team.
Having said that and taking into account this fusion is using artifacts too, I don't see how the teams are winning. Not in a forum fight that is, I'm sure in a comic book the heroes will find a way. But not in here.