1. Borderlands
Before Borderlands, few people had even heard of Gearbox Software, though they were instrumental in bringing several Valve games to life (or Half-life). Nevertheless, Borderlands was released alongside several huge (and established) titles, including a new Call of Duty and Uncharted 2. No one was really keeping their eye on this new IP with cel-shaded graphics.
How did it succeed against the odds? Borderlands was a sleeper hit that steadily gained widespread success based on its own merits. People fell in love with the online co-op mode, ability to replay the campaign seamlessly and seemingly infinite weapons that made the game ridiculously replayable. Before we knew it, Borderlands became one of the top games of the year and was even considered by Gameinformer to be one of the top 200 games of all time.