Dota 2 is a free to play cross-platform MOBA.
Pro Incredibly deep gameplay
For players who put the time into Dota 2 to get past the early beginner phase, Dota 2 can become an extremely satisfying game. There's a reason it's one of the most popular eSports titles in the world. The amount of strategy, skill and variables involved make it a very deep game where anything could happen.
Pro It's very well-balanced
Gameplay is balanced in the sense that while other MOBAs would have only around 30-60 viable picks for the competitive scene, Dota 2 has heroes and strategies in the game that are more varied, which enables more heroes to be picked in the competitive scene.
To compare between Dota 2 and LoL pick diversity, LoL has 64 out of 131 champions picked for LCS.
There are also factors such as the ban/pick phase difference and the more rigid positions in LoL that may limit the viability for different playstyles and hero picks (typically it is 2 (AD, Support Bot) -1 (AP mid) -1 (Tank Off) - 1 (Jungler), with a possibility of double junglers in LoL).
On the other hand in Dota 2, there are a lot more different positions compared to Lol/Smite such as Duo Mid (1-2-2), Traditional (2-1-2), One Jungler (1-2-1-1, most similar to league), One Roamer (1-2-1-1 as well with the difference being the hero roams the map for kills to create space for the mid and safelane rather than farm the jungle which can weaken a lane), Duo Roamer (1-1-1-2, all lanes are weaker in lane presence with the tradeoff being they get solo experience with two aggressive supports roaming), Trilane (1-1-3, Aggressive or Passive Trilane) and Abandoned Offlane (0-1-3-1 where the offlane is completely left empty due to a strong duo or trilane on the opposing team).
More heroes are viable and balanced in Dota 2 partly due to the greater flexibility in laning. This can be attributed the mechanics such as creep pulling and creep stacking which allows the team doing trilane to make up for the shared experience between three heroes. Besides that, the existence of teleport scrolls that allow you to defend your towers or punish a tower dive also allows you to react and assist other lanes.
In pub games, any hero/lane/lineup can work, and it is more about your individual mechanical skills (understanding of hero, combo execution, map awareness, last hitting, timings, item builds, flow) over the lack of coordination and teamwork (on both teams).
Pro A very diverse MOBA
This game has a steep learning curve, but it's well worth it. The games that came after Dota like League of Legends, Heroes of the Storm, Smite, and other lookalikes don't even compare to the complexity of Dota 2.
Every hero is unique and valuable in one team composition or another. The amount of versatility of each hero is amazing as well. The mechanics by far destroy any other Dota style game (MOBA) and that's what makes this game top tier in its genre. This game is the highest grossing eSport game in the world and will probably continue to be for a long time.