Few things are more fun than dropping onto the table out of reserves and shooting - especially because a gun's range is a lot more predictable than a charge roll. Chaos Space Marines have a couple of nasty shooting options with built-in deep-strike capability - Terminators and Obliterators. The problem is that you're probably only going to have room for one of these. So, of course, the follow-on question is 'which one?' Given the versatility of terminator weapon loadouts, and the random nature of fleshmetal weapons, it's not the easiest question to answer.
UPDATE - had a buddy remind me of the stratagem 'honor the prince' - Emperor's Children terminators become a sub-species of "probably get stuck in after shooting" which is more fun.
Given the amount of math and space involved, I'm going to break this into two posts - the
Obliterators are here.
Terminators, The Short Version - Combi-plasma and cheap melee is your most reliable loadout. Save CP for Veterans of the Long War and Endless Cacophony, and you can theoretically delete a Knight or other superheavy in a turn, and also guarantee that you're going to see most of the other army pour into your terminators.
Also, sorry World Eaters - you lose out on a couple of crucial pieces of this and just aren't going to be as effective, but you got Red Butchers, so lean into that instead.
Baseline Assumptions
No unit performs in a vacuum. You're going to be hitting the field on turn two with some high-powered guns - so you're going to bring support. By and large these units want the same thing, so we'll assume that they have it. What is 'it', you ask?
1) Assume we give them Mark of Slaanesh so we can use the Endless Cacophony stratagem and shoot twice.
2) Assume we have an accuracy buff in there - either Prescience for +1 to hit, a nearby Chaos Lord to re-roll 1s, or possibly both - maybe you got a Demon Prince into position. I kind of like the Lord/Prince aura because the only way to turn it off is to kill the guy, whereas psychic powers can fail or be denied.
3) Assume we are willing to drop Veterans of the Long War if we need the +1 to wound.
4) Legion Trait - I'm using the Alpha Legion trait because, y'know, I'm Alpha Legion - so we force a -1 to hit penalty against shooting if you're more than 12" away. I'll discuss other Legion Traits in an addendum.
Terminators Part 1 - Loadout
I lean towards plasma guns and chain axes on terminators, with a chainfist or two in a larger unit. For 390 points you can get a 10-man unit with combi-plasma, two chainfists, and 8 chain axes.
Why plasma guns? You have access to the following guns for terminators -
Combi-bolter - fine for bargain-bin distraction terminators (26pts for combi-bolter/chainaxe - not bad) but it's a totally different roll.
Combi-flamer - you're deep-striking out of range. Pass.
Heavy Flamer - see above, and you get 1 per 5 terminators.
Reaper autocannon - great range and volume of fire - four shots at S7, AP-1, 1 damage - but you get one per five guys, and you take a -1 to hit when you land. Skip.
Combi-melta - here's our contender. S8, AP-4, d6 damage? That's pain. Note that you get one shot at 12", though - you better hope you hit - you are vulnerable to screening (especially against units like Primaris Infiltrators and their 12" denial bubble), and odds are that you won't be able to utilize the 'shoot twice' stratagem with the melta profile unless you went after some HUGE target and/or had choice positioning.
Note also that as long as you have an accuracy buff, you're really competing with the plasma gun's overcharge mode - an extra AP at this point is mostly academic. AP-3 or -4 are either up against an invulnerable save, a 5+ armor save max (outside of cover special rules), or you're going clean through it. When it comes to damage, 2 damage and volume of fire wins against basically all infantry compared to melta shots, but the meltaguns do have the potential to out-damage plasma guns - they'd need a 5+ damage roll to beat both shots in rapid-fire range, and outside of 12" they're just sad.
Combi-plasma - Here's our money-maker, thanks to its range and versatility. You get S7, AP -3, 1 damage, OR you super-charge for S8, AP-3, 2 damage. You have a 24-inch rapid-fire gun, so in a perfect world you drop at 12" and unload, supercharging if necessary.
Note also that the 24" range means that you can still contribute if someone does still have deep-strike denial in play, OR if you nuke your first target and still want to abuse Endless Cacophony, you have options.
Why chain axes?
Really it's easy - let's keep it cheap - the chain axe / combi-plasma combo runs you 35 points a guy. You're not here for melee, but you're probably going to end up in it. Giving everyone power weapons is at least another 30 points or so - and understand that after the first round of shooting, no one wants to let these guys do it again.
I do recommend a chainfist on the champion (go ahead and abuse his 3 attack profile) and maybe one more in a max unit, just in case you have to punch something big to death.
Terminators Part 2 - The Damage
So, we have our ten terminators with combi-plasma. Let's assume we drop in rapid-fire range, and get Prescience off. That's twenty shots, and with a 2+ to hit we're going to land 16.66 shots. Damage wise we should consider stuff with and without invulnerable saves.
Vs. T3 infantry OR T4 infantry with overcharge - Veterans of the Long War (VoTLW) is irrelevant
Wound on 2+ - 13.88 wounds
Saves vs AP-3
4+ or worse in the open - 13.88 wounds go through it
3+ in the open - 11.5 wounds go through
3+ in cover OR 2+ in the open OR 5++ - 9.2 wounds go through
2+ in cover OR 4++ - 6.9 wounds go through
Vs. 3++ saves - 4.6 go through
The above math applies for T5-7 targets as well, assuming you use VotLW to get down to a 2+ to wound again - and it's very much worth it.
Next, let's look at the hardest targets - T8. You are ABSOLUTELY using VoTLW so you can wound on a 3+ here with your shots. Anything else is just a waste.
Wound on 3+ - 11.11 wounds
4+ or worse in the open - 11.11 wounds, 22.22 damage
3+ save in the open - 9.25 wounds, 18.5 damage
3+ save in cover OR 2+ in open OR 5++ - 7.4 wounds, 14.8 damage
2+ save in cover OR 4++ - 5.55 wounds, 11 damage
3++ save - 3.7 wounds, 7.4 damage
Just double that for average output w/ Endless Cacophony. The good news is that you can do a serious number on large targets - a good deep strike, Prescience and 3CP will probably kill you a knight.
Terminator Perks -
-Plasma is pretty nasty.
-Plasma is pretty reliable, and S8 hits important break-points versus heavy targets.
-You do have the ability to split up fire in a fairly granular matter if needs be and you have the appetite for math.
-You can focus a LOT of buffs on one unit fairly easily.
Terminator Drawbacks -
-You HAVE to drop within 12" to get the most out of your guns.
-You probably blew up 300+ points of the enemy army and you're like 12" from it - life's about to get rough.
-You have a pretty reasonable footprint - you really want guns on the ground turn one to clear a screen.
-If you don't get a to-hit buff or a way to re-roll ones, you're asking to blow up terminators.
A Note on Legion Traits
Alpha Legion - you aren't necessarily going to benefit from the legion trait based on your 12" range, but if you get charged and LIVE, you have the stratagem Feigned Retreat to fall back and shoot for 1CP. Expect anyone who knows about this to make an effort to severely weaken you and tie you up in melee, if not kill you.
Black Legion - the trait is irrelevant. You can get some re-rolls, though, via stratagems - Let the Galaxy Burn is 1CP to re-roll 1s. Tip of the Spear is 1CP to re-roll hit rolls for a Black Legion unit as long as it's the closest the enemy, which you probably are.
World Eaters -
Skip. You lose out on Endless Cacophony because you're stuck being marked as Khorne, and you lose Prescience because you hate psykers. If you want terminators, run Red Butchers as S5 base melee terminators.
Word Bearers - Legion Trait is largely irrelevant to the terminators. Of note is the stratagem Malevolent Covenant, which means you auto-pass a psychic test (Prescience, anyone?) for 1CP and a mortal wound on the psyker. It can't be subject to Deny the Witch, either.
Night Lords - Legion trait is again irrelevant. The stratagem In Midnight Clad is 1CP for -1 to hit vs. shooting, which can actually help. Prey on the Weak is 1CP for +1 to hit IF your Terminator's Leadership 9 is higher than the target's - this can free up VotLW for someone else, potentially.
Iron Warriors - Ignoring cover is situational as a legion trait, but worth remembering if you're hunting heavy infantry in cover. You do have some stratagems worth remembering - Iron Within, Iron Without is a cheap 'ignore wounds on 6+' and Dour Duty reduces the AP of enemy melee attacks, which may help you live through the drop. Tank Hunters is about your only real damage buff, but gives you a re-roll to wounds vs. vehicles.
Emperor's Children - your legion trait is more about punching than shooting. However, for 2CP you could use Combat Elixirs to make your terminators T5, which might be funny. For you, anyway.
EDIT - perhaps of more importance is the stratagem 'Honor the Prince' - for one CP, one dice is a 6. For your charge. Can you get one 3+ on 2d6? If so, then for one CP, you're in melee, kids!
Renegades - the Scourged get overwatch on 5+, which may help. Otherwise, nothing really jumps out.
In Closing
The combi-plasma alpha strike is pretty sick - save it for hard targets, and make sure you clear a drop zone 12" away from the enemy to abuse it. Save CP for Veterans of the Long War and Endless Cacophony. Your legion may or may not offer much extra, but as long as you can get accuracy buffs and clear out ~500pts of your list for the terminators and an HQ, you're good.