MOBA or Multiplayer Online Battle Arena is a genre of games that pits two teams of five players against each other on a map with three lanes surrounded by forest or jungle. The main objective is to destroy the opposing team's main structure, so the number of kills can be irrelevant. As it relies heavily on teamwork on coordination, this type of games can be hard to learn and master for new players; however, if you're interested, here are the best free MOBA games.
League of Legends
League of Legends is the most popular MOBA game currently on the market even though it has some particularities that I don't necessarily like. For starters, you only have access to a few of the available champions and the rest can be either unlocked through in-game points or with real money. Nevertheless, there is a weekly rotation of free champions, so you won't get bored. Another aspects are runes and masteries which give small in-game advantages, but that also require in-game points or money to unlock and that means that people with money or who have been playing for a longer time will have an advantage over "the new guy". As far as the gameplay itself is concerned, LoL has a faster pace than DotA and it's a bit more reliant on individual skill.
Valve's take on DotA, the original Warcraft III mod that launched this entire genre, relies only on 13 million players per month, but its competitive scene is the strongest one in the world, completely reshaping e-sport and offering record-breaking cash prizes. Even though DotA 2 also includes microtransactions, they are purely cosmetic, which means that money won't bring you any in-game advantage. What's different from LoL is that in DotA you can last hit your own creeps, "denying" them from your opponents and you can use a courier to bring the items you bought to you. The game is also a lot more teamwork-based, which means that it's the hardest one to learn for new players. In my experience, the DotA 2 community, although not friendly at all, is still not as terrible as LoL's.
An innovative take on the MOBA genre, Smite doesn't display the action from the top down; the player's perspective is similar to the one from third-person shooters (non-isometric) and you actually have to aim your attacks and skills in order for them to hit the intended target. Another cool thing about this game is that it features gods from various ancient pantheons. The concept of the game is the same and so is the objective, but the change in perspective makes a world of difference.
Heroes of the Storm
Blizzard's take on the MOBA world, Heroes of the Storm features heroes from multiple universes including Warcraft, StarCraft, Overwatch, Diablo and retro games. What's great about this title is that it's the easiest one for new players who are on their first take on the genre, it doesn't feature as many heroes, the action is a lot more straightforward and matches take less time. However, just like in LoL there's a weekly rotation of 14 heroes that are available for free and you're going to need to unlock the rest yourself or wait until they're in the rotation themselves.
Heroes of Newerth
One of the oldest MOBA games on the market, Heroes of Newerth is very fast-paced, which means that a new or inexperienced player may have a hard time figuring out what's going on on the screen. Just like in LoL the players from a team can decide to surrender through voting, but unlike any other game of this kind, Newerth also offers the option to vote for kick a player that is AFK, intentionally feeding, trolling or being toxic.
Lastly, Paragon is a MOBA that's still in its beta stages, but which looks pretty good. The game is more similar to Smite than any of the other titles, showing the action from a non-isometric perspective. It includes a few features such as cards that can be added to the characters offering various passives and bonuses through the game. Just like any other free to play MOBA, it also includes microtransactions and from what I could figure out, they can actually influence the game itself. However, I'm going to wait until I have a clearer picture before I start criticizing its developers for choosing the "pay to win" strategy.
Unfortunately, even though MOBA games have grown from a small niche to a global phenomenon in just a few years, there aren't too many new titles. It's very likely that the genre is pretty restrictive, so coming up with innovative ideas is quite difficult. However, the titles on this list, especially the first three ones are still among some of the best co-op games that you can currently play.