I might as well start off with the obvious one on this list. Terraria, to date, has sold a monumental 18 million copies across all platforms. At its heart, Terraria is a 2D Minecraft, but it goes a few extra miles to create an identity of its own.
While it gives the player the ability to build anything they want in 2D, its gameplay also features RPG mechanics. You will need to defeat monsters, explore dungeons and face bosses to gain better equipment and progress in the game.
You need to explore various biomes and build a town with various NPCs that you will have to protect from monster raids. With over 1,000 items including crafting materials, weapons, armor, clothing and spells the possibilities with Terraria are near endless.