In the fictional Warhammer 40,000 setting, the Tyranids are a nomadic alien race comprising many genetically engineered forms. They are the "Great Devourer" and are the "most alien race" in the Imperium. They are an "elemental force" that seeks to consume all in their path, draining all planets of any possible resource with horrific speed.
Tyranids were first described in Rick Priestley's Rogue Trader, the first edition of the Warhammer 40,000. At that time they were not an emphasised race in the game, instead representing a limited number of occasionally-encountered alien antagonists. In later editions the Tyranids became a playable race in their own right, popularised by a number of successful board games. Unlike most Warhammer 40,000 races, the Tyranids do not have a Warhammer Fantasy Battle counterpart, although they possess reptilian characteristics, similar to the Lizardmen & many of their larger units resemble dragons.
Tyranids are typically a close combat army and mostly rely on swarming their enemies through numbers.
The Zerg Swarm are a race of fictional insectoids and the overriding antagonists of the StarCraft series. Operating as a hive mind, the Zerg strive for genetic perfection by assimilating "worthy" races into their own, creating numerous different strains of Zerg. Unlike the Protoss and the Terrans, the Zerg do not use technology, instead using assimilation of other species and directed mutation to develop traits to match such technology. As with the other two primary races, the Zerg are the subject of a full campaign in each of the series' real-time strategy video games. Zerg units are designed to be cheap and fast to produce, encouraging players to overwhelm their opponents with sheer numbers. Since the release of StarCraft the Zerg have become a video gaming icon, described by PC Gamer UK as "the best race in strategy history". The phrase "zerging" has entered video gaming jargon to describe using many low-cost and weak units to overwhelm an enemy.
Who would win?
...Personally, I vote for the Tyranids. <s>Because the Zerg ripped them off.</s>