Depends what you are fighting against really!
Having a decent number of Lascannons or similar to deal with armour is seen as more or less mandatory. But if you have this covered elsewhere you could look at other options.
Autocannons are pretty good, especially against elite infantry like Primaris. Anything with 2 wounds hates Autocannons!
Plasma cannons are pretty bad on Dreads in my opinion. They're basically Lascannons with randomised shots, less damage, less range, and a risk of hurting yourself. I really don't see any reason to take them over twin Lascannons on a dreadnought.
Also consider it's generally a good idea to match up ranges and desired targets on a single unit as much as possible. So Lascannons don't really compliment an Assault Cannon because the Assault Cannon forces you to move up the field and find light vehicles and infantry, while the Lascannons want to sit back and snip vehicles. So the Lascannons are definitely better with a missile launcher, whereas if you go Assault Cannon you could consider doubling up or even going with the fist/scourge.
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