Viral Flash games are capable of driving massive amounts of traffic, and building brand awareness or collecting opt-in details. While the style of game will inevitably depend on the brand or product to be promoted, there are certainly some genres which are better suited than others.
Puzzle games, word games, driving games… in fact most genres can be made to work well as a viral game, but the most essential element is that of competition. High scores which can be posted to Facebook or Twitter updates encourage other people to play the game and attempt to beat their friend’s scores or achievements.
Another option, rather than using scores as the hook, is to use creativity. By allowing the player to create something which can be shown off to their friends it’s possible to create the same sort of lure to get their friends to play. From dressing up celebrities, to building snowmen, or painting cars, the idea is always the same – give the player the creative freedom to draw/build/create something which they can show their friends and encourage them to create their own.
A third method for hooking players and encouraging the viral spread of a game is to include some personalization. By allowing a player to add their friend’s names (or even profile photographs) to a game, it’s possible to massively increase the temptation for their friends to join in. Most people will be intrigued if they receive an email or social media message, personalized to them, inviting them to take part in a game that their friends have recommended.
So as you can see, the actual genre of a game is probably less important than making sure it includes elements which encourage sharing and competition. Certainly viral games should be kept simple, and appeal to as wide an audience as possible (within the target demographic) but rather than concentrating on genre, a better approach is to ensure that whatever genre is used, the game encourages viral spread.