Terminators are the most powerful infantry unit of the Space Marines. They are an elite ranged unit and are proficient in melee. Storm Bolters are the default weapon but Heavy Flamers and Assault Cannons are optional upgrades. Terminators can also be upgraded with chainfists and cyclone missile launchers. Terminators have high health and armour, but this comes at a price of slow movement speed, which can be solved with Teleporter research, however. Terminators also aren't as good in melee as they are with ranged so it's a good idea to make sure they're protected by melee or ranged units so they don't get caught in melee. Of course, you could just teleport them away but that ability does need to recharge.
Production site: Chapel-Barracks.