It will depend slightly on each person's tactical approach but I'll run through my thoughts on the units I've used. I'd be interested to see how others incorporated them as well.
Army design/approach: DW for is in-your-face assault with specialty kill teams, supported by the transports, flyers or heavy support (landraider).
- Reivers. Most flexible units as they are cheaper than a kill team, they can deep strike AND they have 2 wounds. If you need to fill 100 pts, include a squad of 5 Reivers and grav chutes. It has become an auto include for me. Best used on other infantry types, elite melee infantry might give you trouble.
- Repulsor. Godly expensive transport unit, but the only transport available for Primaris. All of my opponents have targeted it first and usually succeeded in killing it by the end of turn 2. I kitted mine out to be a firing platform since I can shoot heavy weapons at full BS on the move. My thoughts for it now is to kit it out with an auto-launcher (smoke), advance turn 1, pop smoke and try to shoot turn 2. If you aren't transporting primaris units, do not bring it. Bring another Blackstar or more razorbacks.
- Primaris Captain. He's not horrible. I would use him with a heavy stalker bolter to support my stalker/missile team perched up on terrain. Even if you move him around, he can do some good (BS2+ with -1 for heavy weapon and master-crafted). But honestly for the roll I use him, a watchmaster is a better option. Captains are more of a front line support if you are already incorporating a watchmaster. But if you want him on the front lines, how do you transport him?
Potentials (stuff I haven't used):
- Primaris Apothecary. Awesome when paired with (expensive) kill teams, probably most useful on the front lines. Elite choice but still a character. Transport is a problem. Maybe roaming stalker bolters (Hellfire rounds) models paired with some special weapons could be used to maximize range since they'll be footslogging.
- Primaris Chaplain. One extra wound and a better pistol. Transport and footslogging is the biggest issue. Maybe you could transport him in a Repulsor with some Reivers to maximize his abilities?
- Hellblasters. Bought a set but haven't used them yet. Awesome support unit on paper, able to deal with multiple types of threats. Their range is pretty good for the default guns (30", rapid fire, AP -4!). Their price is also comparable to a kill team (and 2 wounds) but you'll likely have to replace or downsize an existing kill team in your army to make it fit. Transport is less of an issue for these guys in the support role but they have great assault potential against multi-wound tanks/MCs/super heavies. I can't recall if the heavy version of the weapon gives more range or not. I don't think it did... but that would be a reason to take it.
- Aggressors. Good potential but you've got to get them across the battlefield. They're a little expensive when trying to use them with DW. Better potential in a regular space marine army.
- Intercessors. Good and more wounds but I'd rather have kill teams or reivers.
- Interceptors. I'd rather have more kill teams or reivers. We're already an "elite army" and point management is very important.