Ultimate Hero?

Slark - just learn to land Pounce and you can start pwning everyone with ease, especially if you have stun/slow support like Crystal Maiden (which is amazing, it's even possible to make double kill at first minute with her). You can find some nice builds on this site.

Clinkz is also very easy to learn, just get that lvl 6, eat big creep in jungle and go for double kill. Killing heroes in early-mid game with him is just too easy to fail. When you make crit item, the only thing you should be afraid of is long stun and gem, cause everyone will die in few shots if you are lucky enough to land 1/2 crits. It's also very easy to run from one lane to other if you use build that prioritize E over Q.

Lina is another hero that should be mentioned, she was one of my first heroes I was learning and I know that she is very powerful even in mid-late game due to her ulti doing 1250 damage at lvl 3 with aghanim and very fast attack speed (I had best results maxing Q first, E second and W last, cause W is sometimes too hard to land and buff from passive is much more reliable).

Sniper - nothing more to say, unhittable in laning stage if you make use of his attack range, also projectile speed makes last hitting and denying incredible easy.

It sounds like you want to play gankers/semi-carries - so yeah Night Stalker is a great example of one. The biggest difficulty you'll probably finding is that you're trying to change role as well as change hero - the people you've been playing as are carries: farm, get occasional kills, win late game. Gankers are a different breed, your power comes a lot earlier, but also starts to disappear as the game goes longer - you need different skills and a new mindset.

The long version...

Well, to be honest, most people would suggest getting the "1 button" heroes such as sniper, skeleton King and so on.
It's true, these hereos are very good at every level of the game.
Night Stalker is great early and mid game but hes not that great in the late stages of the game.

Lifestealer is an awesome answer to your thread though hes usually played passively until he gears up a little. Visage is also awesome. Warlock is useful all game too because of the Fatal Bonds in the early stages to harass people and after that he's useful because of both Fatal Bonds and Ultimate Skill. His support and babysitting capabilites are also good in the laning phase.

However, the best late game hero (assuming he's farmed properly towards his optimal item build) is defenately Faceless Void.
This hero can literally own any hero with or without use of the Chronosphere. (Tested against most mellee carries. He owned the mighty Lifestealer in a 1v1 same items full item build scenario.)

Surprised no one's mentioned Ursa, no one in pubs seems to be able to play against him and he does ridiculous amounts of damage very quickly.

there is no ultimate hero, DotA is A team based game so even if u pick a "good" hero and your team F***s up, u lose.

How come Pudge is never mentioned? With his Meat Hook into Dismember and Rot combo, he is very strong early to mid game.

Because he's really easy to counter and a risky pick. Talking about competitive matches.

Hmmm... So true. But he does own in pubs, especially when the player is very skilled.

I have found Juggernaut to be easy to play, and with a lot of the builds on this site he can be very effective throughout the whole game. Lifestealer is great once you learn how to jungle. Sven and Drow Ranger are both strong carries, so if you feel comfortable with Drow Ranger that is honestly a good hero to try to master.

I would consider my two mains to be Juggernaut and Lifestealer, and I am intending on starting to learn Disruptor, Dazzle, Luna and Skywrath Mage.

There's just so many viable heroes in this game! Find 2 or 3 you like, even if you don't do that well with them, and just practice. Real matches, bot matches, everything. Just have fun! It's a game, after all.

You could read a bit about Razor, then training a few matches against bots and putting your skills to the test against real players. It is a great hero to deny and last hit, as well as to get some kills during the whole match and making the enemy carrier useless!