I've always felt Thanos' durability and damage soak is really what makes him shine against foes of his weight class and beyond. It's more impressive to me than his energy blasts or punching power.
Debating for or against Darkseid is difficult imo, as you could just as easily try to use a special amped version or "arc" version of Darkseid to support your claim as you could an avatar or a less formidable version of Darkseid to do the same.
That being said, I do think Thanos has the edge in H2H more often than not, primarily due to his absurd level of durability and damage soak.
Personally I find old gravel-face to be the superior due to skill. IMO he has faced more skilled opponents than thanos in h2h. Just my personal opinion. I give Thanos the edge in damage soak though.
Darkseid is an amazing villain...imo, he IS the beat villain but DC does him so bad that its hard to see him beating people who tier he "should" be in. Thanos wins this but if we use any version of Darkseid in the 80's and 90's, I would give him a high majority.