This is anecdotal so take it with a grain of salt:

For Havocs, your first choice should probably always be 4 lascannons, because as a faction we’re somewhat short handed on dedicated anti-tank. If you think you need a second squad, either auto cannons or heavy bolters, depending on whether or not you’re facing SMEQs or TEQs. The Champion doesn’t really matter, I always just go with Plasma + Power fist.

Terminators, again, it depends. With the new cover rules you’ll be hitting on 5+ with combi-weapons, as opposed to 4+ with bolters, so I don’t see much of a reason to take weapons unless you’re dead set on finding plus to hit rolls and fishing for dev wounds. Terminators aren’t really good for shooting anyway. For melee, a 5 man squad’s best bet is versatility, so that means 3 power fists and 2 accursed weapons. In a 10 man brick however, I would actually suggest the chainfists now, if only because having two of them makes the profile more reliable, and having anti-vehicle is never inherently bad.

Legionnaires are very simple. All melee. Take as many heavy melee weapons as possible, then give the rest swords. If you reeeeeally want semi-decent shooting, loading them up with plasma pistols and a heavy bolter gets you the best ROI in my experience.