1. Necrons
According to the community and also from my experience, the strongest faction is the Necrons. It may take a long time to become truly powerful, but until then you rarely run the risk of being overrun.
Your earliest units, the Necron Warriors, are very slow but nearly unbeatable and absolutely free. With a few troops, you should be able to fend off most attacks until you reach the late game and build your strong units here.
In particular, your commanders and the later ability to turn your base into a superweapon that can single-handedly destroy entire bases makes Necrons the strongest race in almost every match-up.
Unlike all other factions, Necrons do not require requisition points but only energy for everything. Control points and the like increase the building, training, and research speed. The system is unusual for beginners and takes some getting used to, but with some practice, you should not have any problems.
Additionally, the unique ability of Necrons to revive themselves is notable. Instead of first having to produce troops in the base, most warriors simply stand back up where they died. Some other units can teleport directly to the battlefield.
The greatest weakness of the Necrons is their slow movement. All other factions move fundamentally faster, and you can almost kite any Necron unit, meaning: harass from a distance and run away when it gets too close. A strong rush is also a good tactic against Necron players.
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