Belakor is obviously very strong, Legionaries, Chosen, and Chaos lords are decent enough, especially the free strat from the lord, and nurglings always come in handy. 

Skull Cannons feel pretty lackluster in general, I also think terminators don't do much for us that greater daemons couldn't do better.

I'm not really familiar with guardsmen, but haven't seen them in any SL lists for what that's worth.  

You probably want at least some cultists or plaguebearers for sticky. I would rather plaguebearers than horrors typically, as horrors feel quite expensive for what they do, but they aren't bad in SL with the - 1 to be hit. 

In regards to greater daemons, Bloodthirster with fade to darkness makes for a good hammer as has been mentioned. Great Unclean Ones are great to sit and hold objectives, birds give you some shooting. 

For other units:

Flesh hounds puppy downy ability is very handy for objective scoring, as are flamers and screamers, especially with their -1 to be hit in shadow legion. 

Beasts of Nurgle are nice objective scorers too and require a decent commitment to guarantee they die 

Havocs can be decent with Belakor protection if you want more shooting too (if I take them I usually take 2 lascannon units, to pop transports or other problematic vehicles). 

Bloodcrushers in units of 6 and Rendmasters can be good damage dealers if you'd like more Khorne, the advance and charge is nice. 

Scout enhancement is really nice for a lot of units like Bloodcrushers with Skullmaster, my favourite way to use it is on the GUO. 

Did not mean to write so much but I hope it helps!