assault squads with lots of flamers
Contrary to popular belief, flamers aren't that great against hordes, sure they have a template weapon, and what, they'd fire that once or twice before being bogged in close combat for pretty much the rest of the game? Instead, you could have a bolter, fire a few turns, rapid-fire and statistically kill more orks. Maybe in Tactical Squads where they're free, but otherwise I wouldn't. Really, bolter drill comes to mind, the str makes it easy to wound (we already hit on 3s) and the AP is just right, unless the enemy spends points on toyz to prevent it, in that case, they have less bodies to sustain fighting.
Whirlwinds work wonders, 'nuff said. Vindicators can also work against the nobz, the high str, low ap and large blast make it perfect, it can also double as simpleton troop killing aswell (although things with better saves have higher priority in the vindi's books).
Combat Tactics to pull out of assault and rapid-fire the remaining orks to pieces, safe gamble seeing as space marines can't be wiped out due to sweeping advance.
Vindicater vs Whirlwind
It's range vs. firepower, one of each will do fine.
Plasma weapons are almost silly against a race that the base save is 5+/6+,
although they can take down the big 'uns, the chances will be few and far between, seeing as HQs will usually hide in big mobs of nasties, and those big mobs of nasties will get cover saves from the screening mobs. Rendering AP2 near useless.
although marines are pretty decent at assault, they are better at shooting
Space Marines are the jack-of-all-trades, master of none
The best part of our race is our flexibility, and how easily we can send another squad to do a different role, won't see that in too many xenos armies.
Edit: Read that the other way around, previous point still stands however.
A unit of Sternguard vets would really help add to the firebase, and you can give them decent close in weapons too.
Doing that will make the squad too expensive, it's a specialist squad trying to do a different job, give them bolters, leave it at that.
Also, a starting point for what tactics you usually employ helps. A list works wonders as well.