Busting ghosts, hunting monsters, and breaking bricks.
Starting with March, we're going to start concluding each month by voting on the best game released on a Nintendo system (and possibly on others as well) during that time span. We'll be running something similar in the forums, too (check out the poll for March), and we'll post your choices and arguments.
Our overall winner for March was Luigi's Mansion: Dark Moon, but since this is more of a trial run, we're including arguments for a few other frontrunners as well.
Neal Ronaghan - Luigi's Mansion: Dark Moon
My excitement for Luigi's Mansion: Dark Moon was based more on developer Next Level Games' pedigree than total affection for the original game. Fortunately, Next Level Games delivered totally and completely, taking everything that was great about the original and refining it to a fine, addictive adventure. While the level-based progression was a little worrying at first, it works because you can still explore the environments to your heart's content. It just works better for the portable format now.
What truly surprised me was the multiplayer, though, as the cooperative experience is a ton of fun. Whether it's local or online, I think I'll be coming back to Dark Moon's multiplayer experience quite often over the coming months. This was definitely my most-played game in March, and it's also my favorite of the month.
Tyler Ohlew - Luigi's Mansion: Dark Moon
The reason behind my choice for Luigi's Mansion: Dark Moon is simple enough, I simply had the most fun with it. Sucking up ghosts is just the start; it wasn't long before I craved every rug and strip of wallpaper. Despite its limited moveset, Dark Moon always manages to keep things fresh with its puzzles and hidden trinkets.
What makes it so interesting is how it compares to its predecessor. While aspects are missing, it creates new points of interest to call its own. While I certainly adore Monster Hunter 3 Ultimate, nothing else entertained me as well as Luigi's Mansion: Dark Moon.

Zack Kaplan - Monster Hunter 3 Ultimate
Being my first Monster Hunter game, Monster Hunter 3 Ultimate (3DS) has convinced me that I have been missing out on a lot of fun all these years. I choose it as my game of the month, but I will definitely be playing it beyond March for its mix of engaging gameplay and addictive quests.

Guillaume Veillette - Lego City: Undercover
Say what you want about developer Traveller's Tales and how tired Lego games based on popular licenses are at this point: they have acquired over the last decade an expertise for injecting personality and humor into their games' worlds. Given how important it is for an open game to have an interesting world, this expertise pays off in Lego City: Undercover. Every nook and cranny of Lego City has something cool to see, or some humorous easter egg to find. The player has to complete the main story missions to earn the abilities that will allow them to explore the game fully, but good writing and endearing characters ensure that they'll want to do so anyway. The family-friendly title is very easy for experienced players, but the platforming and driving both feel good enough to be enjoyable for their own sake.
Nate Andrews - Luigi's Mansion: Dark Moon
If nothing else, Luigi's Mansion: Dark Moon makes a nearly unassailable argument for Luigi as the more interesting brother. Secretly, the man in green has texture and personality for days, and Next Level Games' excellent translation of the series to the 3DS brings it out perfectly.
I'm enjoying the level-based progression (justified gripes about the lack of a convenient save system notwithstanding), the way the game captures the mischievous intimacy of each area, and the fairly slick control scheme. It's a game that creates interesting spaces to poke around in, and gives you all the tools you need to have fun doing just that.