The standard
LRBT works wonderfully against all sorts of Marines. Against loyalist marine it tears em up, and even negates the advantages of Chaos Plague Marines by doubling their toughness. If you get one of the upcoming Demolisher Leman Russes, you can even wreck Terminators.
Autocannons are great to take (I'm guessing that's what you meant by "assault cannon" ones
) as they can just inflict a lot of wounds with relative ease, and even do some damage against vehicles. With loyalists you can do some damage against dreadnoughts, rhinos and the like, and against Chaos you'll be able to blast away at daemon princes, greater daemons, and anything with Mark of Nurgle.
Veterans with plasma guns can do wonderfully against both flavors of marine and their respective Terminators, and the aforementioned Plague Marines. Melta guns are nice too, and can hit dreadnoughts and land raiders really hard. Could infiltrate them or put them in Valkyries.
Keep things cheap so you can outnumber Marines. You should ideally outnumber them by a load. Try not to go to crazy and get loads of Stormtroopers - the AP3 is real nice, but they cost a buttload of points.
Apologies for the long post, but there's a lot to know about fighting Marines! Just load up on high strength, low
AP weapons, and drown them in bodies!