I think Darksied wins this.
Thanos has lost to characters weaker than him like Spiderman. Thanos may have beaten Silver Surfer and Thor, Darksied beats Superman-level characters like Orion.
Speed is obviose. Thanos is strong, he is really strong while Darksied's omega beams can keep up with Superman and The Flash and another thing about the Omega Beams that it can track it's opponents even if it's turning corners.
Durability is close since Thanos has great durability, Darksied can tank attacks from Superman and Supergirl. Thanos does have an impressive durability feats like tanking Odin's attacks, Darksied on the other hand has an insane regeneration helped him out a lot in his faces and Thanos does have regenaration though it is not very effective against Darkseid.
The biggest factor is intellegitents, Darksied has eons of experience and is smarter than Thanos. Darkseid has way more experience than Thanos and Darksied is not afraid using dirty Tricks.
And Thanos's weaknesses can be a major risk seeing that he can subscoisely loses himself and sometimes make mistakes.
So the winner is Darksied.