Thanos went toe to toe with Odin, blasted and actually singed perhaps even hurt Galactus, has whipped Silver Surfer's pansy ass on numerous occasions, and in my opinion Silver Surfer would beat Darksied and Superman,beat a more powerful clone version of himself, has overcome death and is practically immortal all without the Infinity gauntlet. Thanos may have started off as a lame ass carbon copy of Darksied, but he has become so much more. Except for the lame Kieth Griffin incarnations in the the Marvel Annihilation story arch. Drax killing Thanos, nahhh, after I read that I didn't either bother to finish the series to see if it was just another clone or not.
Thanos just cause he's more interesting to me. It's not that Darkseid isn't interesting, it's just that Thanos has so many more dimensions to him. That and he is a complete screwball which is always a plus in my book.