Resident Evil Mercenaries is one of the best games I have played in a long time. It is like a little adrenaline rush every time I play.
I've been playing tons of Mercenaries on the 3DS over the past two weeks and I love it. The first few tutorial levels are a bit tedious but once you get to the levels in section 4 it is nothing but thrills.
Resident Evil Mercenaries: The Basics
The levels are taken from previous Resident Evil games but they are perfect. And the game is balanced very well with a low enough supply of ammo where you can't be reckless and enough enemies where you feel like you should be.
Resident Evil 3DS Multiplayer
And there are always people ready to play. I've played at 9am on a Saturday and 11pm on a Thursday and almost every time in between. I've found a partner within 10 seconds every time.