Devastators are the most straightforward and iconic of the Space Marine Heavy Support choices. A squad of Marines that can take up to four heavy weapons, Devastators provide a stationary firebase. I prefer to take them as a full ten Marine squad, as the extra bodies protect the heavy weapons and the ability to Combat Squad allows them to engage two targets and cover more of the table.
The main question with Devastators is what heavy weapon to give them. I always suggest taking four of the same weapon, as this means they have the same target preferences and range. Missile launchers are a common option because they can target both infantry and vehicles fairly well. Flakk missiles are an option here; they are quite expensive but they do allow your Devastators to reliably shoot down fliers. If you don’t have any other anti-air and are taking Devastators, it’s worth spending the points for flakk. Lascannons are your best option for anti-tank, while plasma cannons will chew through heavy infantry very well. Heavy Bolters are an option, but there are generally better options for anti-light infantry fire (we’ll get to it in a bit). Multi-meltas, while great weapons, don’t do their best work on such a stationary platform due to their short range. You will be better off taking them on Attack Bikes or Speeders.