There are a few reasons why they all chose to take the form of Harry, but these are entirely explained in the books as opposed to the films.
I'll start with my own speculation, before moving on to evidence from the book. I believe if they disguised him as, for example, his cousin Dudley, it would look especially suspicious. Imagine if Voldemort and the Death Eaters had seen a collection of wizards, then this random boy that they know Harry lives with and doesn't like? That would make him stand out even more. The same logic applies if they were to disguise him as a random muggle from the village, or wizard from the wizarding community (not to mention they need some "part" of whoever they want to change in to).
On top of this, even if this ruse was successful, if the real Dudley was seen in future he would likely be killed instantly by the Death Eaters as they would suspect it was Harry. In other words, it would draw unwanted attention to Harry's family, putting their lives at danger. Again, this logic applies to any other wizard/muggle they could disguise him as.
They could have made Harry change into another member of the party, e.g. Hermione or Ron, but this solution isn't really any different to what they actually did in the end. Doing this would have resulted in two or more Hermoiones/Rons and would have left the same problem for the Death Eaters that their actual solution (the Seven Potters) did.
Therefore, their final solution of disguising everyone as Harry seemed to be the safest solution - albeit, still very risky (as the death of Moody showed).
Now, turning to the books as evidence - firstly, from Snape's pensieve flashbacks:
“You will have to give Voldemort the correct date of Harry’s departure
from his aunt and uncle’s,” said Dumbledore. “Not to do so will raise
suspicion, when Voldemort believes you so well informed. However, you
must plant the idea of decoys; that, I think, ought to ensure Harry’s
safety. Try Confunding Mundungus Fletcher. And Severus, if you are
forced to take part in the chase, be sure to act your part
convincingly. . . . I am counting upon you to remain in Lord
Voldemort’s good books as long as possible, or Hogwarts will be left
to the mercy of the Carrows. . . .”
Now Snape was head to head with Mundungus in an unfamiliar tavern,
Mundungus’s face looking curiously blank, Snape frowning in
concentration.
“You will suggest to the Order of the Phoenix,” Snape murmured, “that
they use decoys. Polyjuice Potion. Identical Potters. It is the only
thing that might work. You will forget that I have suggested this. You
will present it as your own idea. You understand?”
“I understand,” murmured Mundungus, his eyes unfocused. . . .
This shows that it was Dumbledore and Snape's idea - so Dumbledore himself really had thought this gave Harry the best chance of escape. Ths strategy essentially served two purposes - to give Harry the best chance of survival and to firmly establish Snape's reputation as Voldemort's most trusted ally.
The books also describe why the solution of apparating couldn't work. As explained in the seventh book:
Moody dropped his sacks at his feet and turned to Harry. "As Daedalus
probably told you, we had to abandon Plan A. Pius Thicknesse has gone
over, which gives us a big problem. He's made it an imprisonable
offense to connect this house to the Floo Network, place a Portkey
here, or Apparate in or out. All done in the name of your protection,
to prevent You-Know-Who getting in at you. Absolutely pointless,
seeing at your mother's charm does that already. What he's really done
is to stop you getting out of here safely.
"Second problem: You're
underage, which means you've still got the Trace on you."
"I don't -"
"The Trace, the Trace!" said Mad-Eye impatiently. "The charm that
detects magical activity around under-seventeens, the way the Ministry
finds out about underage magic! If you, or anyone around you, casts a
spell to get you out of here, Thicknesse is going to know about it,
and so will the Death Eaters". "We can't wait for the Trace to break,
because the moment you turn seventeen you'll lose all the protection
your mother gave you...We're going to use the only means of transport
left to us, the only ones the Trace can't detect, because we don't
need to cast spells to use them: brooms, thestrals and Hagrid's
motorbike."
So any attempt to transport Harry would have resulted in his whereabouts being immediately known - and as we see a few lines later on, the group really believed they had misled Voldemort and thought they could safely get Harry out:
"The one thing we've got on our side is that You-Know-Who doesn't know
we're moving you tonight. We've leaked a fake trail to the Ministry:
They think you're not leaving until the thirtieth."
So ultimately, they couldn't apparate without drawing attention to themselves. Harry then turns seventeen at the Burrow. This conversation takes place at the Burrow:
Hermione: "What if the Death Eaters have found a way to put [the
Trace] on a seventeen-year-old?"
Ron: "But Harry hasn't been near a Death Eater in the last
twenty-four hours. Who's supposed to have put a Trace back on him?"
So Ron, from a wizarding background, seems to suggest the Trace needs to be manually reapplied. Therefore, after his birthday, they could apparate safely, but not before.
