As a Death guard player I must say so far spawns and bikes didn't impress me, whereas plague marines were always the main workhorse. Similar to Necron warriors they don't die and can fight against anything. They are also much better than
CSM +
MoN making the
CAD still important despite Traitors Hate.
Bikes, Raptors and Spawn are fast but that's it really. They don't kill things, sorcerers and Lords do. Lord with a Daemonheart is especially fluffy and also our only source to 2+ armour aside from termis.
Also, sorcerer with Balestar of Mannon in a unit of obliterators worked really well for me. Due to prescience or other divination powers you really make them a strong shooty unit and if you get close prescience will also help you in
CC.
Havocs are always solid, don't expect them to outshoot anyone though. They are supporters, but they do it well and for cheap.
Can't say much about the Daemons other than soulgrinder with
MoN or Plague Hulks (they are cheaper) are nice distractions and won't die when parked in some terrain.
Unfortunately Grotty the Nurgling also lowers the T of
MoN units, so keep that in mind. However, a herald could be used to generate some warp charges, or maybe you are lucky on the improved plague table.
Can't give you a list here, but maybe these hints help you

.