Number Battle is probably my favorite dsiware game. It's a strategy board game where you take turns placing numbered tiles on a grid. You earn points by forming different types of combos, based on the number on the tile and the direction of the paths on the tiles. One easy example of the strategy involved is if you place a 3 tile which has 3+ paths (let's say the paths go E, W, and S), and then build a 4 and 5 sequence on E and W paths, when you place a 2 on the S side of the 3, you'll get a combo for 2-3-4-5 twice, as game counts 2-5 going E and W. If there are additional branching paths off of the 4, you can get even more points for one play. Then you play the 1 on the 2 and you get even more points.
There's always room for turnabout though. Some of your tiles will be rainbow colored, indicating that other players can build off of your tiles (something that's usually not allowed). Rainbow tiles leave you open to have your combo blocked, but they also give you mobility. Non-rainbow tiles can only be placed adjacent to your other tiles (regardless of whether the paths connect or not) but rainbows can be placed anywhere. This means you can also place a rainbow anywhere on the board, to block the enemy. Add in items like ones which add/delete paths, change blocks from rainbow to non, land mines, etc, and you have a ton of depth.
I never played it online. I guess the dsiware MP servers are down now, or are being taken down soon. The campaign (which I'm still playing) seems to be long enough to justify the price (so far I've played ~15 levels), plus you can also go back to get gold medals.