Thanos vs Darkseid... This is a really tough one. Both are master planners with an intellect that makes Doom and Richards look like scientific amateurs. Darkseid's Omega beams are impressive, but Thanos' teleportation tech as well as his own resilience and energy manipulation powers help even the playing field out. I give Darkseid the overconfidence flaw, and all Thanos has to do is exploit that to ensure a win. A tough call but I give it to Thanos, who's outsmarted the devil among other impressive feats.
Dr Doom vs Lex Luthor... "Bah. Doom cares not about such things as money. All the money in the world will not save you from Doom's wrath". We're talking about a normal human (very smart, true) vs an individual who's at least as smart garbed in one of the most sophisticated suits of battle armor on the planet and can also wield magic almost as well as Dr Strange. I almost feel sorry for Lex.
Green Goblin vs Joker... Again, we're talking about a normal insane guy dressed like a clown vs a superhuman insane guy armed to the gills with high tech weaponry. Who do you think would win?
Ultron vs Metallo... I've got no clue what Metallo's made of, but it had better be at least half as durable as True Adamantium if Metallo is even going to stand a chance.
Taskmaster vs Deathstroke... If Taskmaster has had time to study Deathstroke's fighting style, he'd win after a hard fought battle. Otherwise, Deathstroke is just too much for him. Taskmaster is after all only human, while Deathstroke is as close to Captain America as DC has.
Sabertooth vs (who I assume is) Gorilla Grod... I don't know anything about Gorilla Grod, but he's going to have to be pretty mean and strong to take on Creed.
Red Skull vs Ras Al Ghul... I honestly don't know enough about Ras to know which way this would go.
Magneto vs (no clue who that is)... Again, no idea.