Even after his brutal scorching in the fires of Mustafar, sand remained the only natural phenomenon that Vader feared. To him, sand was a reminder that he'd failed to save his mother or Padme from death.
Darth Vader doesn't fear the Emperor, he despises him, just as Vader so vehemently despises himself. So, if not Palpatine, who could Vader possibly fear? Well, the answer is deceptively simply, yet incredibly shocking all at the same time: Luke Skywalker.
Darth Vader Hates Palpatine More Than Obi-Wan (& Himself)
However, by the end of the issue, it's strongly implied that the true source of Darth Vader's hatred was Palpatine, and he used that hatred to crush this Scourge-host with the clenching of his fist.
Darth Vader's Insecurities are His Biggest Weaknesses
In the end, virtually all who have made the attempt to use Vader's cybernetic nature against him have ended up dead or defeated. The elite few who have had the upper hand, even before his conversion to mechanical monstrosity, have done so by dominating him mentally.
Darth Vader was so feared because it didn't matter who, or how many, or where he faced anyone. Darth Vader destroyed and murdered his way through every situation that got in his way. Indomitable, inexhaustible, unfeeling and ruthlessly efficient.
Having faced his father once before, Luke was familiar with his tactics and used the force to gain an upper-hand. In Star Wars Canon, only Luke Skywalker can be acknowledged as the one who truly defeated Darth Vader.
In a Vanity Fair interview from 2005, George Lucas went deeper into why Anakin became much weaker as Vader. In the interview, he explained that because Anakin's limbs were severed and replaced by robotics, it was harder for him to use the Force than it was before his battle with Obi-Wan.
Darth Vader possessed the skill to use Force lightning, but the limitations of his suit prevented this. A lot of the systems that were keeping Darth Vader alive were maintained by electricity, which is why Force lightning was so destructive towards him.
Vader had been using all his energy to intimidate Luke, so when the actual fight began in a flash, he was already weak and panting. This leads to another huge factor, which is the age difference. While Luke was a young and spry 23-year-old, Vader was reaching his late 40s.
Anakin does not have even the slightest bit of remorse. By the end, he feels nothing but anger. Although he feels regret for killing Padme, he still helps the evil Emperor. The film ends with Anakins standing side by side with the man on whose behalf he has committed all his crimes.
The canonical comic Star Wars: Darth Vader #7 confirms the younglings' deaths caused Anakin grief, self-loathing, hatred and pain, fueling his descent even further into the dark side.
While Vader had his weaknesses, including vulnerability to electricity, characters like Emperor Palpatine, Mace Windu, Yoda, and Obi-Wan Kenobi all had the potential to defeat him in battle.
Obi-Wan Kenobi soundly defeated Vader in Star Wars: Episode III - Revenge of the Sith and Obi-Wan Kenobi, and is easily one of the most powerful Jedi Vader ever faced. Vader and Obi-Wan were similar in swordsmanship and Force-sensitivity, but Vader's hatred of Obi-Wan often caused him to fight irrationally.
Despite being equally as strong in the Force, one innately Sith ability that Jedi in the Star Wars franchise do not wield is Force lightning. First introduced when Darth Sidious (Emperor Palpatine) electrocuted Luke Skywalker in Star Wars: Return of the Jedi, Force lightning is a powerful weapon for Sith.
Either way, while Vader can use most other Force abilities, Force Lightning needs to be channeled, and Vader's limbs are explicitly not connected to the Force. The Star Wars: Episode III - Revenge of the Sith Visual Dictionary confirms Darth Vader cannot use Force Lightning, and even suggests this is the reason why.
Darth Vader can not use force lighting, because of his severed arms. Robotic hands can't summon the force lighting. However Darth Vader can still use other forms of the force, such as force choke.
Kylo and Rey (Daisy Ridley) also blasted the Imperial Crusier open trying to Force pull the Lightsaber away from each other, so clearly, Kylo Ren is an extremely strong Force-wielder. Nonetheless, it can be said that Darth Vader is the more powerful master of the Force for one key reason: his temperament.
With enough training, Rey would even be able to overpower him. Though Vader may have been the "Chosen One," being born from the Force, the power that Rey was born with was arguably stronger.
The realistic answer seems to be, yes, Anakin in his prime could have defeated Luke Skywalker (in the current canon) if they battled one-on-one. The main reason behind this is Luke's lack of formal Jedi training, at least when it comes to combat.
Although Thanos has a higher intellect than Vader, the Dark Lord of the Sith is wiser regarding battle strategies and war experience. Despite this, Thanos' raw power should not be underestimated, even against the strongest of the Empire's tactics. Thanos has survived against Galactus' strongest attacks.
Yoda's presence on Dagobah was likely hidden due to his mastery of Force Stealth and the planet's connection to the Force. Yoda, with his enhanced Force abilities and potential home-field advantage, would have had the upper hand against Darth Vader if they engaged in a lightsaber duel.
