To start off, I suggest taking a look at this list that ranks each and every Dota hero by "difficulty" to play, based on six main areas of Dota "knowledge":
- Hero/Item Knowledge
- Map Awareness
- Micro Management
- Survival Priority
I would say that this list is a nice guide to reference if you are looking for heroes to play based on your various areas of familiarity with the game (for example, you can sort heroes by their map awareness rating if you think you aren't very good at it).
In general, heroes with an escape mechanism, a stun or disable of some sort, and aren't very farm dependent make for the easiest heroes to pick up and play in my opinion. These heroes tend to usually be Intelligence heroes and often play a support role, although there are some exceptions.
Venomancer: An Agility support hero that has a multitude of skills that damage and/or slow enemy units. Venomancer's wards can allow you to push/farm a lane pretty safely, and his passive slow and gale can be used to gank enemy heroes pretty effectively. Venomancer is not very farm dependent, and his ultimate can do devastating damage to enemy heroes. Make sure to safe his Poison Nova ultimate for team fights, or at times where you can hit multiple enemy heroes at once (and have someone to help you take them down). Like the other heroes mentioned below, he has poor HP and mobility.
Vengeful Spirit: An Agility support hero with a decently ranged and damaging stun, a low cost armor reducing "wave" spell, a damage aura, and a situational ultimate that swaps the position of you and your target. In team battles or during ganks, you should be focusing on stunning an enemy and reducing their armor with your wave. Her "Swap" skill should be used to initiate ganks, or to pre-empt a team fight by taking out one of the enemy heroes to give you an advantage. Since Swap can potentially put you in danger, however, you need to be careful when to use it. You can also use Swap on an allied hero to save them, although you may end up sacrificing yourself as a result. VS's weaknesses are low HP, mobility, and a low mana pool, although getting Mana boots on her can solve that issue.
Crystal Maiden: She has a stun, a slowing nuke, and an aura that gives global mana regen to you and all heroes. A very useful hero that can support and set up ganks when coming from the jungle, or just a hero that can harass the enemies in lane while babysitting (protecting) a carry. Her weaknesses are her low HP, slow movement speed, the fact that she has to stand still while casting her ult, and the fact that her abilities don't scale well into late game. That being said, even if you yourself are having a bad game and end up getting underleveled, her aura still makes her useful to allies.
Lion: He has a stun, a hex, a mana draining skill, and a very powerful nuking ultimate. The fact that he has two disables makes him incredibly useful as a lane ally as well as in team fights. Like Crystal Maiden, he has the same weaknesses such as slow movement speed and low HP. He also has mana issues, since his spells have large costs. Depending on how you build him, however, you can sustain your mana reasonably. For example, skill impale and mana drain early. In lane, you can harass heroes by impaling them followed by casting mana drain on them. Alternatively, you can harass with impale, then use mana drain on the ranged creep to replenish mana a little bit more safely.
Lich: Another support that has a slowing nuke similar to Crystal Maiden's, a Frost Armor skill that lets you buff allies with armor, a skill that lets you sacrifice an allied creep to gain mana, and an ultimate that is a nuke that bounces between any nearby enemy units. Like CM, he can be useful to set up ganks or to babysit carries, and like Lion he can sustain his mana by using his creep sacrificing skill. He has low HP and can have mana issues. His ultimate can be hard to aim correctly as it is a situational skill, but when in doubt, you can always cast it on an enemy hero as an extra nuke, or just cast it on a hero during a team fight.
Skeleton King: A tanky carry that has a stun, a passive critical, a life leech aura, and an ultimate that uses mana to resurrect him should he die (the ultimate has a cooldown so you can't just keep resurrecting over and over immediately). Like yx. said, you really only have one skill, which is your stun. Use it to chase down enemy heroes, or to get yourself out of a tight situation such as a gank. His passives make it very easy for him to kill jungle creeps. He suffers from a low mana pool, slow movement speed, and low mobility in general (after your stun, many heroes are capable of running away from you).
Sven: A tanky carry similar to Skeleton King in the fact that he has a stun (except this one hits in a small AoE around your target). His stun is one of the best level 1 stuns in the game, and can be very useful when trying to get a First Blood or initiating ganks. He has another active skill that increases the movement speed and armor of you and nearby allies, which is useful for closing in on enemies or for running away. His ultimate adds a large number of damage to your regular attacks for a period of time, which is extremely useful for pushing down towers or in the middle of team fights. While farm dependent and has a low mana pool like Skeleton King, his natural tankiness and passive cleave skill makes killing jungle creeps easier.
By no means would I say that these are necessarily the best heroes, but picking them up and learning how to play them correctly lets you familiarize yourself with other aspects of the game, and make the transition to more intermediate and advanced heroes easier later along the road.
As a final note, the heroes I listed above are considered EASY TO PLAY but are not necessarily EASY TO MASTER. All have some sort of slow or stun in their skillset, allowing you to stay relatively safe throughout games. These are heroes that allow you to work on fundamentals without worrying too much about things like "what playing a support means", or "how to be good at last hitting" or "what items you should buy". There are optimal ways of playing each of these heroes (which you are unlikely to do when first starting off on DOTA), but these heroes' skills put less of an emphasis on that, and allows you to be immediately useful to your team.