After reading all the answers, I think I can now make my own conclusion about the question.
Kakashi vs. Itachi
In the anime, Itachi is certainly one of the most powerful shinobi. It has been observed that Kakashi was defeated by Itachi by the Tsukuyomi. This is one of the strongest jutsu that he could use. But as has been remarked earlier, Sharingan user can't protect himself from a Genjutsu cast by Mangekyo Sharingan. To remain immune, the opponent should at least use Mangekyo Sharingan (other than the method that Jiraiya said to Naruto, which applies mostly to the people with an immense amount of Chakra). Kakashi was seemingly unaware of his Mangekyo Sharingan at the time he fought Itachi, so he failed to defeat Itachi.
It should also be noted that the Amaterasu would have no effect on Kakashi, because he could simply warp away the flame or move into the Kamui dimension. As for the Tsukuyomi, Kakashi's Susanoo would perfectly counter it.
Besides, as Sakurai Tomoki noted in his answer, Kakashi can perform all 5 basic elements, which is a feat Itachi is not known to have. In this aspect, Kakashi is also ahead of Itachi. Itachi also noted that fighting Kakashi alone was dangerous with a high chance of injuries.
Kakashi vs. Jiraya
While comparing between Jiraiya and Kakashi, it should be noted that in the anime, Kakashi was in a developing stage, meaning that his power was increasing whereas Jiraiya's power showed no notable development.
In terms of Taijutsu, Kakashi is surely ahead of Jiraiya, in contrary to Sakurai Tomoki's answer. Let me quote the relevant paragraphs from Jiraiya's article on Naruto Wiki:
While not being his preferred combat method, Jiraiya was still very proficient with taijutsu, capable of stopping some of the individual dogs that split from the Giant Multi-Headed Dog just using his legs. His natural strength, while nowhere close to his team-mate Tsunade, was very impressive. In the anime, there is one such occasion where this is shown, when Jiraiya intervened in a fight between a young Gamabunta (who was still bigger than himself) and an unknown snake of equal size where Jiraiya grabbed and tossed the snake a great distance away with ease. Also in the anime, Jiraiya, as a child, was able to lift a large boulder several times his size. Jiraiya has showcased a high level of durability over the course of his life; taking a direct kick to the face from Might Guy, as well as surviving a direct blow from Nagato's Giant Ox, which ploughed him through several steel pipes, without getting injured. Even at the brink of death he still managed to figure out Pain's secret and engrave it on Fukasaku's back, so he could deliver the intel to Konoha.
This is all about Jiraiya. Now for Kakashi:
Kakashi is very adept in taijutsu. In the anime, as a child, he was skilled enough to hold his own against his sensei, Minato Namikaze, in his bell test, managing to touch one of the bells during the test. As an adult, his skills improved considerably, able to easily defeat hundreds of enemies using well-coordinated attack patterns to handle the multiple enemy strikes. Kakashi can also fight on par with Might Guy, a taijutsu master, ever since childhood. Likewise, from his various challenges against and mission with Guy, Kakashi has copied and developed considerable skill in his friend's Strong Fist-style and several of its techniques, enough to train others in its usage. He is also noted to have impressive speed; able to intercept an opponent faster than unfocused eyes could follow even from a distance.His speed allows him to effectively catch his opponents off-guard to quickly strike them down and easily evade attacks. Even against highly experienced assassins, Kakashi can silently sneak-up on them, making him prime to strike. His speed increased considerably in Part II; before the timeskip he had difficulty keeping up with Itachi Uchiha, but he out-manoeuvred him after the timeskip. Kakashi has also shown remarkable reflexes, able to instantly react and effectively manoeuvre out of surreal danger.
Kakashi's speed is further seen during the Fourth Shinobi World War, able to enough to outmanoeuvre a Version 2 jinchūriki despite its enhanced and multiple fields of vision, and even keep pace with Might Guy using six of the Eight Gates. Kakashi was even able to blitz, and nearly sever Kaguya Ōtsutsuki's arm in one move, despite having her multiple dōjutsu active. Likewise, his hand seal prowess is very great, able to perform them at highly difficult speeds to follow. He can also open at least one of the Eight Gates, a feat that very few shinobi can achieve and, in the anime, utilise advanced techniques derived from it while showing no signs of fatigue afterwards. His raw strength is also noticeably high, able to clash with a physically strong opponent like Zabuza Momochi, who was wielding the rather heavy Kubikiribōchō, simultaneously throw Naruto and Sasuke a large distance, snap one of Nagato's chakra receivers with his bare hand, and later easily wield Kubikiribōchō himself with a single hand for extended periods of time. He's shown a rather impressive level of durability and endurance, able to survive Nagato's building busting Shinra Tensei from upclose with minimum damage and continue fighting afterwards.
user3275697 remarked that Jiraiya fought Pain in his full glory, while Kakashi wasn't able to defeat even one Path of Pain. True. But the difference in Jiraiya and Kakashi's situation is that Jiraiya fought with the Paths with Gamaken and defeated them in his Sage Mode, while Kakashi fought the two Paths of Pain without even using his Mangekyo Sharingan. Therefore, no conclusion can be drawn from this fact.
Except for Jiraiya's Genjutsu, it should be noted that Kakashi has the ability to encounter all of Jiraiya's attack. For example, he can easily warp away Jiraiya's Rasengan, which he used in his battle against Pain.
Besides, I personally think that if Kakashi were to fight with the Paths of Pain whom Jiraiya fought against, he would be at an advantage because of his warping ability. He could simply warp the summoned animals into Kamui dimension, while Jiraiya could not.
Also, Kakashi is certainly ahead of Jiraiya in terms of Ninjutsu. He is said to copy about 1000 jutsus.
Kakashi vs. Orochimaru
It is really very hard to compare them. However, Kakashi has improved a lot since their first fight (note that he didn't know how to use the Mangekyo Sharingan at the time of their first fight) and nothing can be said with certainty. However, one thing that is worthy (perhaps) to note is that in the statistics between Orochimaru and Kakashi, Orochimaru stands at 35, while Kakashi is at 34.5
Kakashi vs. Tsunade
In this case, I think that Kakashi can easily surpass Tsunade in every other cases, except for reserves of chakra. But nothing authentic can be concluded, so I go with user3275697's answer.
