Besides smash, I recommend Monster Hunter 4 Ultimate. Other people already mentioned why though, so I'll instead talk about other good multiplayer games. Firstly, Resident Evil Revelations and Resident Evil: The Mercenaries 3D. If you've ever play The Mercenaries mode from last RE games (not to be confused with Mercenary mode), you know what to expect. It's a survival mode basically, with a time limit that's constantly ticking down. You can earn more time by meleeing enemies and smashing time crystals. However, this is expanded to a full game, with missions that can be played coop. It's fun, but I recommend Revelations over The Mercenaries 3D. As spinny mentioned, it has an addictive coop mode called Raid Mode, where you're playing missions that have been taken from the main campaign, but altered some. You level up characters, get randomized loot, deal with special enemies, etc etc. In a weird way, it's like the RPG elements from borderlands, but with Resident Evil. Then there's Fantasy Life, which can be played coop. It's an addictive little game, but beware, because if you buy the origin islands DLC, you can't play with people who don't have it, and vice versa. There's also Kid Icarus: Uprising, which has a more traditional multiplayer mode where two teams fight each other with guns and swords, until enough people die on one team that a bar fills up. Once the bar is filled, the next person to die becomes Pit, or Dark Pit depending on the team. Then you have to take out the enemy team's pit to win, but they're powered up so it's harder to do. It's really fun ^.^ Finally, I recommend New Super Mario Bros 2. You can play coop with a friend similarly to how people play multiplayer on new super mario bros wii, except this one is just too player. You can also exchange ghost times for the addictive coin rush mode. There's other games I could mention, but this is already big, so I'll stop here
TL;DR: I recommend monster hunter 4 ultimate, super smash bros for 3ds, fantasy life, resident evil the mercenaries 3d, resident evil revelations, kid icarus uprising, new super Mario bros 2