I think the Leviathan works pretty well for any Space Marine chapter as it is brutally effective. For my money, the best loadouts are either Storm Cannon and Grav Flux Bombard or dual Stormcannon (prefer one of each but some people prefer maximum dakka

).
As for complimenting them, Leviathans are very tough but do tend to attract firepower so keeping an Iron Priest nearby to patch it up is not a bad idea.
I would also keep some infantry nearby for screening. You do not want the enemy getting into
CC with a Dakka Leviathan since even if they don't do much damage, they will rob it of a turn's shooting.