this game
It's the hardest game known to man.
Not "hard" per-say, just "hard to get into".
Though Etrian Odyssey (and its sequel) on the Nintendo DS, those two games are HARD. To even stand a chance of surviving the first few floors of the circa 100 floor dungeon, you have to put hours of time and effort into the game. If you have a lot of free time to waste, it's well worth it, because it's one of the best dungeon crawlers on the DS. But if you don't have a ton of time to grind, prepare to die A LOT. Seriously, your party WILL die, the death WILL be extremely painful for all involved, and you too will know the annoyance of monsters known as "F.O.Es".
In Japan, the game is so popular, it's spawned a fan-made remix album, which features a song called "F.O.E", which is about just how annoying these F.O.Es are.
This is the (subtitled) music video of aforementioned song.
Edit- oh yeah, it's worth noting that this game will, much like WoW, take over your free time and ruin your life if you ever want to complete it. I've played the first game for 150 hours and counting, and am nowhere near "done".