There's quite a few factors on both sides of this, so it depends.
What condition is Galactus in? Is he fully fed? half? starving?
Does Thanos (without IG) get prep?
IG Thanos beats any form of Galactus, with or without prep. IG Thanos is so OP, it's ridiculous. Fully fed Galactus (meaning full power, obviously) is pretty damn OP, but IG Thanos is a different level, especially if he has prep.
Galactus wins the first match but Thanos can still give him a decent battle if he has prep, Galactus still wins that though, IMO - the second match is Thanos all day, with or without prep. IG Thanos stomps bad.
Darkseid is still the coolest, though. I feel he has the better story and is overall more competently-written, Thanos is just too OP to the point it just becomes ridiculous and uninteresting. Thanos is actually just a Darkseid rip off anyway.