I originally wanted to do some comparison groups together that reflect groups of heroes who usually outcarry others...this is kind of in my head, but very difficult to get down on paper.
Trying to break down the situation using stats is of limited help - quite often the stats growth of some hard carries isn't that great - so this is more about passive skills, or extremely potent active right click skills.
This is my "top 5" list, vaguely in reverse order of who I consider the hardest (i.e. best hero with maximum farm and levels):
5.
Morphling is a stats monster, growing strong and able to shift points to where they are most needed. 3 escapes give great mobility, and the ability to make a strong illusion of an enemy hero also helps with scaling.4.
Medusa lacklustre stat growth is compensated by the ability to attack the whole enemy team at once, absorb massive amounts of damage with 
Mana Shield and a good ultimate. Seriously hard to bring down when well farmed.3.
Spectre - built in damage resistance and reflection is huge, making her incredibly difficult and painful to take down once she's got big. A cracking ultimate helps too.2.
Faceless Void - probably the best set of passives in the game, plus a great ultimate. Void can probably beat up most other carries in a straight fight, Timelock and Backtrack both scale brilliantly and cannot be countered with items, unlike many similar skills.1.
Lone Druid - somewhat controversial, but having twice as many item slots as any other hero ultimately increases his presence more than any other hero can match, given enough farm.Do you agree with this list? Who would be your top 5? P.S - this doesn't necessarily mean these heroes could outcarry any others who are not mentioned, there are particular heroes who work well as counters...
Here's some of my honorable mentions:
Alchemist - alchy's fantastic farming speed from Goblin's Greed is a major factor in his success as a carry, but this is all about 
Chemical Rage - it covers his poor stat growth with improved HP, regen and mostly importantly, massively increased attack speed. This allows him to get maximum procs from a number of items, perma-bash and generally be difficult to deal with.Lycanthrope - big scaling boosts from
Feral Impulse and 
Shapeshift stack up to produce a serious amount of extra damage, combined with maximum movement speed, making him extremely hard to chase down or evade.
Lifestealer - 
Feast is an extremely nasty skill, scaling to provide extra lifesteal and damage as enemy total HP increases, plus he has some good skills and stats anyway.
Anti-Mage - huhhh? But surely AM is one of the hardest carries!? Well, he is - he needs plenty of farm to be successful, and is very effective with it. The difference is the sheer speed with which he can farm it compared to other, similar heroes. His only damage skills that scales is 
Mana Void.
Sniper - massive stats growth ends up powering massive DPS with annoying mini-stuns and long range. If you can't get to him, he can take you to pieces late game.
Drow Ranger - very powerful aura and an ultimate that stacks agility, with lots of equipment she can be extremely difficult to stop.
Phantom Lancer - once he has enough equipment his illusions can be become incredibly difficult to fight, let alone the actual hero.
Troll Warlord - a definite contender for the top 5, his ultimate, attack speed and potential for permabash make him an extremely strong carry.As you can see, it's incredibly difficult to try and rank carries - and in a real match situation farming speed, team composition and other factors have a massive effect too. I'm sure many people will disagree with these suggestions or be like OMG HOW DID YOU MISS xxx HERO? Which is fair enough, but just goes to show how difficult it is to provide information on this rather than opinion..!
MEEPO
This bugger have a anti-running skill of 5 seconds, if you throw all the nets at the same target, and his Geostrike will literally kill you even if his Earth Bind doesn't. Especially with Scepter, he can outnumber a team to 10 v 5. ANd his
MORPHLING
I played a game where ol'
NAGA SIREN
The hardest thing about her is probably her skills. Once she get
CHAOS KNIGHT
His
SPECTRE
Global presence. If you continue using
After saying so much, it still sums up to your playing style and team, in the end.
4. Alch
3. Tiny
2. Spectre
1. Void
Of course this list is irrelevant as carries don't carry alone and do different things differently. For example Spectre never does tons of damage to buildings the way a Morph, Alch, PL, or Tiny does, but she takes advantage of a farm differential with supports incredibly.
Alch is entirely reliant on his ultimate but can have six slots at the same time Void has one and a half.
In the current version the "best" carries are Naix, LD, Gyro, and Luna. Note that LD becomes basically useless around 70 minutes as supports start to get 3/4 slotted and the bear dies in seconds.
Will keep looking for ways of doing an article, but may just have to leave the ideas a blog if I can't do it justice!
R-Conqueror