Safe to say, this is the best Warhammer 40K game that isn't a Warhammer 40K game.

The game is best played with others, especially with friends, but there are Solo Builds out there for the introverted Helldivers who want to contribute to SuperEarth's Glorious Democracy without having to interact with anyone.

Helldivers has a unique system called The Galactic Campaign, in which every mission conducted by players contribute to a meta campaign that goes on for from a week to even an entire month depending on the number of Helldivers participating in. Every race of enemies has different traits, giving very awesome gameplay varieties. You have to take into account their strengths and weaknesses to prepare your Helldrop properly.

This game needs a lot of new players, because SuperEarth needs us!
Get it now, even without the DLCs you will still be able to do well, existing players are very helpful and they will carry you to victory if needed.
Good luck out there!