Brix by Blue Orange Games.
The Blue Orange Games I've tried in the past have all been really family-friendly. Fun, requiring some thinking, but easy to adapt to suit different age levels, they make games that are good for the whole family. This one is no different. With Brix, you can play by color or by X's and O's. The fact that some X's are one color and some are a different makes it even trickier to spot when your opponent is about to win. Add that to the fact that every piece has both colors and symbols on it, and it means that you might place the piece that leads to victory for your opponent!
This was a great challenge for our kids. It wasn't long before they caught on and were occasionally winning too, especially if we were distracted! It's a fun game that can be played by pretty much everyone, even toddlers, but has enough strategy to keep even adults happy.