While a huge number and variety of games come out every year, truly educational ones are like diamonds in the rough. How do you find good ones? How do you know what to avoid?
Games that are written exclusively for educational purposes tend to seem like great candidates for amazing educational games, but in the overwhelming majority of cases, they are a poorly veiled schoolwork. They tend to be low budget and have little to no gameplay. If you want an example of one, here is one I found on the web.
The most engaging and informative games are largely concentrated in two genres; Strategy and Simulation. Strategy games are more frequently educational because they tell stories on a much grander scale and tend to give players more control over decisions, and you to know whats happening to make these decisions. Think of it this way, a game about a US spy in Soviet Russia, who receives orders and carries them out, stealing documents or placing bugs, or a game in which you are the head of the US spy network, tasked with catching moles and gathering intel through all possible facets. The latter has more potential to be educational. Simulation games are more educational because they simulate the way the world works. Duh.
Some genres have very little potential for education and can be ignored when you are looking for a good educational game. For one, most exclusively multiplayer games are too action based and focused on balance as opposed to reality. First person shooters are also all action.