A while before I joined this site, I saw a forum dedicated to discussing a battle between one Celestial and Thanos. The forum had the set rules of round one featuring no preparation and round two featuring preparation. I'd like to add my comment since the thread is closed.
I say that without preparation, the Celestial wins the battle. With preparation time allowed, Thanos wins the battle. Here is my reasoning for both.
A lot of people mention the Odin vs Thanos battle being a stomp in Odin's favor, which made me wonder if anyone had even read the comic it was featured in. Thanos was not fighting Odin to beat him, Thanos was fighting Odin because Odin would not listen to invaders bringing his defeated son to his doorstep. It was clear throughout the battle that Odin was trying to kill Thanos while the other isn't true. Thanos actually tanked several of Odin's blast before Odin summoned his spear. He also made Odin stumble around a lot during the beginning of the fight. So, some people saying that Thanos couldn't lay a scratch on Odin are lying. The fight was closer than many people on this bored make it seem. That being said, Odin would have won the battle, even if Thanos was serious about trying to beat him. It's be very close, but Odin Force is too much for Thanos to put down.
The same goes for the "battle" with Galactus. What people tend to ignore is the context of the story itself. Thanos and Galactus actually had equal prep against each other. Galactus was fully fed and was trying to use the Infinity Gems to cure his Hunger. He didn't know he was being manipulated by The Hunger. The blast that Thanos took from Galactus actually depleted a fully fed Galactus enough to make him hunger again. Thanos did not beg Galactus to spare his life, Thanos begged Galactus to listen to him about who was manipulating him.
Again, however, that being said. Thanos was only able to tank Galactus's (and Omega, a being twice as powerful as Galactus) blow due to his shielding technology. Unlike where he relied solely on his durability while fighting Odin. Given all of this, it's safe to say that Thanos is low to mid level Skyfather. As he has better feats than a lot of them. I will argue on that point later, but being limited to a mobile device, my scan posting options are not readily available. So here is how I see it. Galactus is, and generally has the potential to wield more power than any one Celestial (besides maybe Tiamut, One Above All and Exitar), he is however, far less durable, and in my opinion, does not have as advanced technology. However, that too is debatable. Even the weakest Celestials, like the Red and Blue Celestials, are beings I consider above Odin and Zeus. Meaning that if a random encounter would go down, Thanos could most likely use his shielding to successfully prevent the Celestial from killing him. He could probably stagger the Celestial. He would then probably teleport away. However, Thanos does not have the ability to put down any one Celestial based on power and durability alone. Which brings me to my reasoning behind my second choice.
And Red Celestials (if you are including Arishem in that) aren't all weak.
Even if Thanos could tank a Celestial blast. Which I don't know if that is possible. They don't tire like Galactus. They'd just sit there constantly pummelling him with energy until his shield gave out and would carry on when it broke.
Thanos doesn't stand a chance in my opinion. Even Celestials like Eson are no laughing matter.
When we consider preparation with prep masters, it's really hard because of how good they are at winning in preparation is involved. Thanos is one of the most intelligent beings in the Marvel Universe. He has demonstrated this in nearly every comic he is featured in. I even (on comicvine) had a Galactus vs Thanos intelligence debate. I broke it down intelligence into five categories (I will not be going into as much detail as I did there unless asked). Each category has an available 20 points maximum. The categories are: Knowledge, Creativity, Cunning, Strategy, and Comprehension (how much you are able to draw from cosmic senses). Galactus scored: 20, 20, 10, 15, 20. Thanos scored: 15, 20, 20, 20, 10 (base Thanos). Thanos has been shown to construct and operate on Galactus level technology on several occasions (the Synthetic Infinity Gem, Turning Galactus and Surfer into a weapon, Omega, The Time Machine that is capable of destroying entire time lines. Etc). A lot of cosmic beings don't use prep in the same way Thanos does. If Infinity Gauntlet and Infinity War show us anything, their strategy is merely to overwhelm their enemies with extraordinary power. People also assume that any Average Celestial or cosmic being could prep to take down an entire Universe as easy as Thanos did. If that's true, then why didn't Tiamut or the Proemial Gos succeed? Again, because their strategy was just to creatively find ways to exhausted their power to defeat their enemies. Thanos's preparation is different. He tricks, manipulates, traps, confuses, and just plain out thinks many of his foes. It's part of the reason he's gained ultimate power so many times. If I was to think of a way Thanos could use his technology to defeat a Celestial, he could teleport the device that generates black holes directly into it's brain. He could find and convince/ manipulate other entities to distract the Celestial enough to defeat and or turn it into a weapon. Maybe he wants to steal it's energy. Maybe he wants to manipulate it for further schemes involving the entire Celestial race. It's just how Thanos has always been written. Not only by Jim Starlin, but by the writers of Annihilation and Thanos Imperative too. We've seen a powered up Odin manage to de-limb a regular Celestial (I think It was Ziran who he de-limed). We've seen Thor toss Odin's sword through Arishem. We've seen Reed defeated the Celestials on numerous occasions. With Thanos' s strategic mind and his ability to construct ridiculous technological devices, I have no doubt in my mind that Thanos would defeat a Celestial in a prep war.
While a Celestial may not need to recharge it's blast, Thanos has been shown to endure blast from beings like Galactus (Fully Fed) and Omega (A being twice as powerful as Galactus). I feel the battle without prep would go like this. They'd encounter each other, the Celestial would fire a cosmic ray at Thanos, Thanos would block it due to his shielding. He would then proceed to fire a blast at the Celestial, staggering it and or knocking it off it's feet (like Thor has done). While it's distracted he would use his untraceable teleportation technology to flea. If the Celestial pursues, he'd do what he did to Silver Surfer and lead him down several dead ends. Enough for Thanos to go into hiding long enough to hatch a plan.
This is true. Had the Celestials feats not varied as greatly as they currently do. I'd suggest that every Celestial is more powerful than Galactus. I'd also say, if they had maintained such high levels of power and only increased since then, that Thanos without prep wouldn't even escape a Celestial. However, that is not the case.
To the individual who asked what would happen if Thanos was turned into something else by the Celestial. He has complete control over his own atomic structure. Not only that, he has been shown to be nearly completely resistant to matter manipulation. Otherwise both Galactus and the Silver Surfer would have used it on him.
Technically Thanos would lose by self BFR then wouldn't he?
And also even if Thanos tries to fire back at the Celestial the Celestial is still firing at him. It won't give Thanos the opportunity like what happened with Galactus. Remember you've said that Thanos blocks the blast then fires back. But the Celestial won't stop.