I'm not the most experienced player overall, but Death Guard are my main force these days...
Plague Towers are a pain. While I
love the idea of them, using them as a transport can be risky due to the crit effect destroying d3 transported units, instantly bypassing the 3+ save of the Death Guard which is their biggest strength.
For the Terminators, I agree, that's probably putting your Supreme Commander in too much danger. He might be better off in the Retinue with Land Raiders.
Plague Zombie Infestation - while they can be useful as a late-game arrival, they're also exceptionally useful as a tarpit arriving in the first turn and spreading out across a quarter or more of your opponent's lie, stopping their advance - just try to make sure they're far enough back that multiple units aren't giving supporting fire once someone Engages. Being Fearless means that even when (not if but when) they break, they can simply reposition to block a different area!
Plague Reapers - once they get within 30cm, enemy formations will (literally) melt away because they've got so many bleeping guns. The problem is getting them there with a 15cm move and no Marches allowed.
They can also be a bit fragile in assaults, so make sure they're supported by other troops. I've found that adding a Defiler adds some decent assault capability, effectively doubles the formation's firepower from 30-45cm and also helps cover the Death Guard's big weakness...
Defilers/anti-aircraft. Outside of Hellblades (which I've not used), Defilers are your sole source of anti-aircraft, and with AA5+ 2 x 45cm they're pretty good at it. While the Defiler formation makes a nasty little AA bubble, the ability to randomly add 1-3 Defilers to almost any formation in the list can be more useful as you can end up covering most of the table. Note that due to their 20cm move, Defilers themselves cannot garrison, but if a single one is added to a slower formation then that formation can still garrison. Note that Infiltrator gives the Defilers a double move in an Engage order.
Plaguehounds - similar to the Plaguereapers in that they can make entire formations melt away once they get close enough, they also have the speed to get there. Just watch out for armoured or transported formations as it will only be the barrage that can harm non-infantry targets. I tend to use 2-3 as mobile supporting units, doubling to take out badly-positioned enemy infantry formations or to soften up a stronger formation before retaining with a Retinue to Engage.
Contagion Engines - they're not in your list but are definitely worth a look both from in-game effectiveness and as a conversion opportunity. With four units in the formation they're nastier than the Eldar equivalent as they're getting two templates of Disrupt nastiness and if you really like barrages you can add a further three to get seven barrage points or even a Plague Tower and potentially hit thirteen freaking barrage points! However as with the Defilers they can be added to other formations such as Retinues, where their 30cm AP4/AT6 secondary weapons can add a surprising amount of punch to what can be a fairly under-gunned formation when it comes to ranged firepower.
