I understand this question invites discussion.

What I meant to ask was if due to

basic stats (which I am unaware of)
a certain item unlocked throughout the run specific to the character
some other mechanism

one character is technically superior and intended to be used to learn the game and make the runs easier, whereas the others require a better knowledge and higher skill to take advantage of them.

Is there any particular characteristic that makes one of the characters the "easiest" to beat the game with?

2 Answers

There are no superior characters (or "Gungeoneers") in Enter the Gungeon; each one has strengths and weaknesses that give them a varying amount of success depending on your playstyle. So instead of making claims about which character is "best", I'll give a brief summary of each character's strengths and weaknesses.

The Marine

Main pistol has longest range of all "basic" Gungeoneers
Starts with Military Training, which provides high gun accuracy
The active item gives one free ammo drop
Starts with 1 armor, so he can take one more hit than any other character

Summary: The Marine is usually considered the "beginner" character, since his high accuracy and long range make aiming a little easier in hectic fights. His extra ammo crate is immensely helpful to replenish the ammo on your best gun, and the extra armor is a nice benefit as well.

The Convict

Sawed-Off shotgun is good for burst or AoE damage.
Molotov is also good for AoE damage, provided you don't step in the fire yourself.
Enraging Photo compensates you when you get hit by providing a little more damage
Both her starting pistol and the shotgun have somewhat short range

The Convict seems to specialize in doing damage to multiple enemies at once. Her short range is offset by the fact that her damage is amplified significantly for a few seconds after getting hit. I personally find her to be the most challenging of characters, but she has a lot of power behind her.

The Hunter

Crossbow can one-shot most of the weak enemies for floors 1-2.
The Dog gives free pickups (ammo, keys, hearts, blanks) (average about one or two items per floor)

The Hunter excels at clearing out the weak enemies so she can focus on killing the tougher baddies. Her crossbow does a decent amount of damage, so it's also not bad for bossing. Her dog provides much needed resources as the game progresses.

The Pilot

Can carry two active items at once
Items in shops don't cost as much (about 15% cheaper)
Trusty Lockpick has a 50% chance to open any locks without using a key.
- If the Lockpick fails, you cannot open that lock, even with a key.
Starting pistol is short-ish range and not very accurate.

Ah, the pilot. I have a love-hate relationship with this character. Because of his lockpicks, he can potentially open a bunch of chests that you would normally skip, meaning you will have more guns/items available to you, making you that much stronger. On the other hand, failing to pick a lock will make a chest inaccessible. His shop discount also allows him to buy a few more items than other characters, further increasing his power. However, the first floor or two is really painful due to his wildly inaccurate starter pistol. This character is a high-risk, high-reward Gungeoneer who scales really well into the late game but struggles early.

The Cultist

Only accessible if you're playing co-op
Friendship Cookie brings one player back to life
Starter pistol has longest range of any other starting pistol

This character is only available when playing Co-op. He's pretty average and makes a decent companion.

Secret Character spoilers below

The Bullet

Main weapon can erase bullets
Does not take contact damage
Can deal increased damage by dodge-rolling into enemies.

This guy is my favorite character because he has a very unique style of gameplay. His main weapon, Blasphemy, is a sword that erases bullets when he swings at them. If he is at full health, the sword will also fire a sword beam projectile (a la Legend of Zelda). If you position yourself correctly, you can hit enemies with both the sword slash and the projectile, doing a ton of damage to them in the process (you can one-shot regular bulletkin up to floor 4). Additionally, his complete immunity to contact damage makes certain enemies (especially jammed enemies) much easier to defeat. However, this strength comes with a price: If this character ever has less than full health (armor not included), his sword will not fire the projectile... and on early floors when you have no other guns, that forces you to fight enemies using melee attacks, which can be difficult for some players. I've seen this character described as simultaneously the easiest and hardest character in the game.

The Robot

Very long range starter weapon
Starts with Battery Bullets, which increases accuracy with all weapons by 50% and shocks any water on the floor
His active item drops lots of water on the floor
Cannot gain health, only armor

In my opinion, this character is the toughest Gungeoneer. Though he excels at AoE damage (due to his active item and Battery Bullets), the fact that he can only gain armor means he is very vulnerable to being hit. You have to be pretty good at the game to be able to win with this guy.

Haven't even played the game so I'm unsure as to why I'm attempting to answer this question, so I've used someone else's answer to the same question, answer is on a Steam Community Form

If your playstyle is cautious and you dislike taking hits, then you should try the Pilot or Hunter. The Convict is buffed if she takes hits and Marine gets an extra armor, which is probably not what you want.
The Pilot has better shops and is good in the long run, but you need to rely on your guns because his starting weapon is trash. He can also pick up two active items, which can be pretty useful.
The Hunter can get extra items (including keys, and ammo) which is pretty nice. She dosen't (sic) get an active but her crossbow one-shots the weak mooks on chambers 1 and 2. This makes her good for the early game.
-Empathy, Steam ID : 76561198041783823 incase you want to thank him or something