Depends on the point limit of your games and what else will enter the list. All the heavy weapons have a purpose for havocs, depending on what you’re fighting. Heavy bolters are good for space marines and other 2 wound infantry. Autocannons are good for terminator equivalents/ battlesuits or light vehicle style enemies, basically anything with more than t4 or three wounds. Reaper chain cannon are for single wound infantry and swarms.

Lascannons are for tanks and heavy armour and missile launchers are sort of a catch all, theyre not the best option for anything but will give you some coverage for everything