Empire versus Federation

I think that either Anakin Skywalker or Darth Vader (taking them as different characters) could defeat Starfleet single handed if he was piloting a B-Wing.

It can be difficult to compare the offensive and defensive capacities of say the Empire and the United Federation of Planets, but I would say it is certain that the defensive systems (shields) used by Starfleet are not capable of absorbing 100% of the energy from most offensive weapons systems.
Similarly, the weapons on a B-Wing (Star Wars: Rebels season two) are comparable to a starship's phasers on a higher setting. Combined with his obvious piloting skill, Vader could easily defeat Starfleet one task force at a time.
A caveat: this ultimately assumes however that Starfleet will not "fight dirty". Considering the wide array of weapons that the Federation is actually capable of using, they could inflict massive casualties on the Empire using "biogenic" weapons, "metaphasic" ships (both invisible and capable of flying through physical matter ) and torpedoes, ships that can fire while cloaked and last, but by no means least, the Genesis Device.

I've been fascinated by Anakin's statement in Phantom Menace that he dreamt he was a Jedi and "came back and freed all the slaves". What if this happened instead and was the crisis/ catalyst that started the Clone Wars and began Anakin's downfall? I suggest we see Anakin as a three-year old (perhaps) then at age ten or twelve then he leaves the Jedi Temple as part of the Trials. Around this time Palpatine is close to the (or head of) the Galactic government. He befriends Anakin and confides that Anakin's mother sold him to the Jedi Order to pay off her debts to the Hutts.

When we next hear about Anakin, the Senate is in uproar over a slave rebellion and commissions an army to either quell the rebellion or retake several hundred systems which seceded in protest at the Senate's inactions. It transpires that Anakin is (one of) the leader(s) of the rebellion which causes Kenobi to be expelled from the Jedi Order (more dramatically he could even turn to the Dark Side).

This scenario perhaps allows the Senate to issue a bounty on Kenobi which is taken up by Jango Fett. Kenobi escapes after killing Fett and orphaning Boba, which leads to him discovering the army of Fett clones purchased/commissioned on the Republic's behalf. Meanwhile, Palpatine contacts Anakin and convinces him that his rebellion is futile. Through a convoluted series of events, Anakin discovers Palpatine is also Darth Sidious. This leads him to renounce the rebellion and convince his followers to re-join the Republic. Meanwhile, Kenobi locates and confronts Anakin, which results in Anakin's permanent injuries and his acceptance of the Darth Vader persona. The reason for this is simply because Kenobi and the Jedi have no knowledge of Anakin's surrender and Palpatine's alter ego; moreover the Jedi would not inform Kenobi of any updates on current affairs while he goes off on his "idealistic crusade". Anakin's rebel ex-slaves join with the surviving clones in forming the bulk of the Imperial military.

As an addendum, Sidious deliberately sets up the Alliance to Restore the Republic hoping (or predicting) that a potential apprentice will kill and replace Vader-either through fearing that Vader will overthrow him or that Vader would soon prove unsuitable as a successor to the Sith legacy.