Right now the rules are designed to work with vehicles as the center point of the army. That means most of the Xeno armies are dead in the water. You'll want to play humans, because they get frequent rules updates, and are always updated within months of a new release. However, the alien codexes languish for years without rules errata or changes or updates. For example: Necron's rules are almost 10 years old now, whereas Space Marines get a new rules over haul almost every other year.
Armies to be competitive: (have vehicles, model selection, consistent rules, stability, multiple in slot selection, and constant rules updates).
Space Marines
Space Marine Variants (wolves, blood angles, etc)
Imperial Guard
Competitive but a VERY high learning curve:
Tau
Dark Eldar
Will sit on your shelf: (no rules updates since 3rd edition, models you can't even buy because they don't exist, no vehicles, stupid rules updates that gut all functionality from the army and remove synergy between units, and most importantly: don't have solutions to the other player bringing a table full of vehicles).
Eldar
Tyranids
Orcs
Necrons
Chaos Marines
Demons
Sisters of Battle