Since ive got time to burn, ill do a rough rundown.
HQ:
Fateweaver: The re-rolls to saves is the main reason for taking him.
Keep him center of the army to keep your units going and use him as a firing platform as he has pretty much every weapon based gift.
Try to keep him out of combat though as he isnt all that amazing when he gets flooded.
Simply keep a unit near by to deny any assaults on him.
Ku'gath: Not a commonly used character as he is very slow, but he is also one of the hardest to kill.
Main reason to take him would be for an epidemius tally list, in which his missile does pretty decent damage.
Other than that though, he is very slow and sluggish, so not great overall.
Skarbrand: A thirster with might an extra attack + breath of chaos.
Also has fleet instead of wings.
So, hard as nails, but his special rule is a real double edged sword.
Against less combat orientated armies its great, but against orks and anything that can swing a weapon you can pretty much kill yourself, more so if its against dark eldar or the likes as they will usually be striking 1st.
Bloodthirsters: A walking battle tank in no short terms.
Throw might on them if you want to make sure you can get the best out of them in tank hunting.
Might also be worth taking the increased
inv. save against psychic attacks.
Keeper of secrets: A slightly tamed down version of a thirster, but more attacks at a higher initiative.
Lord of change: Better save than the other greater daemons and usually act as a firing platform.
However, they stack up on points pretty quickly so your best off going with a daemon prince to do the same job.
Great unclean one: Not much use really.
Slow, doesent hit all that hard, no really "stand out" upgrades for it.
Skulltaker: Worth every single point due to his 4+ rend and instant death.
Either walk him with a unit of bloodletters or throw him on a jugger and put him in a unit of crushers for a pretty nasty unit.
The chariot is a waste though as it stops him joining units and he gets killed very quickly.
The masque: She cant join units, but does have a 3++ and the ability to throw enemy units around 3 times a turn, so she has some use.
She usually gets shot to pieces early on though, so her uses are limited and depend on your play style.
Epidemius: Slow, sluggish in combat and only has a use in a tally list where you will be going all out with nurgle daemons.
Outside of a tally list, useless.
Heralds:
Khorne; Good on a jugger for boosting the strength of crusher units, give him might and rending and he can hunt armour at the same time.
Like skulltaker, the chariot gets him killed rather than helping.
Slaanesh; Alot of attacks, give her might to get her strength up enough to hurt something.
Steed is ok, but chariot makes her last alot longer and unlike khorne chariots, she can still join a unit of seekers.
Nurgle: There really is a pattern here.
They really dont have much use unless you want to add some defence to a unit of plaguebearers on an objective.
Then you simply give him breath to help thin any units that get too close.
Tzeentch: Bolt, chariot, keep him moving and hunting armour.
Out of the heralds, this is one of the most commonly taken ones as daemons struggle a bit with armour.
Elite:
Beasts of nurgle: Remember that pattern that had emerged earlier? its back.
Slow, weak in combat, not worth taking.
Flamers of tzeentch: Normal thing for them is to work as a suicide unit.
Simly use a unit that has an icon to deepstrike them where you need them and open fire.
Dont worry about taking bolt, they wont really live long enough to use it.
The main use is to fry high priority targets like paladins and other termies.
Crushers: Slow, but hit like a ton of bricks.
The icon is a nice option as it provides a stable platform for dropping in units by them.
Instrument isnt really essential, but its cheap and helps you bounce wounds through the unit and last a bit longer.
All in all, a very solid unit, more so when fateweaver is boosting saves for them.
Seekers: They are like crushers, but die alot easier.
They are however stupidly quick, have a ton of attacks and are very useful for grinding through units and tanks.
Again, like crushers, a solid unit.
Troops:
Horrors: The shooting option out of the troops.
Able to lay down a ton of fire, have a better save but are utterly useless in combat.
Still a nice choice though.
Plaguebearers: Mainly used for sitting on objectives since
FNP and high toughness keeps them going for longer.
Dont expect anything else from them though.
Daemonettes: Fast, a ton of rending attacks but very fragile.
Great for unarmoured units like orks (in smaller units) and guard, but if you dont wipe the unit out, you will take heavy losses.
Also good against small armoured units due to rending, but not 100% as rending is very iffy.
Bloodletters: Hard hitting and have power weapons as standard.
Your basic power armoured killing unit in the army.
Good strong choice, but unit size depends on play style.
Nurglings: That pattern is back, need i go on?
Fast attack:
Furies: Like the nurgle choices, they are hopeless.
Flesh hounds: Fast, pretty hard hitting but lack any ability to hunt either tanks or armoured units.
These really are down to a players preferance, but i really do dislike them.
Plenty of other units can do what they do, but better.
Seekers: Like daemonettes, but faster and with more attacks to boot.
Good choice and a great unit for helping in combats due to the speed they can get from different points of the board.
Screamers: I like them.
They move like jetbikes and have melta bombs, so anti-armour is their only real role in the army.
They will usually get shot pretty quickly though, so if you plan on taking them, take 2 units of 6 to get the job done.
Heavy:
Soulgrinder: The only thing in the daemons list with an armour value, so it attracts each and every lascannon, melta and missile in the game.
It does however act as a nasty firebase.
Phlegm is the most commonly used upgrade as the template causes plenty of damage.
Tongue is nice when you read it, but misses most of the time, so your better off with the above.
Daemon princes:
Shooting princes; Mark of tzeentch, bolt, gaze, breath, take your pick here.
Usually wings is a common upgrade along with iron hide, but they start building up in points very quickly.
But for cheapness, mark of tzeentch, wings and bolt will do if your on a tight budget.
Combat: Mark of khorne, unholy might, wings and iron hide will keep them going.
Plenty of speed and hitting power, so tanks and armoured targets are both fair game.
Keep away from large units though as the prince can only kill so much, he will end up getting tied in combat and brought down through weight of attacks.
Another popular build is the same as the last, but mark of nurgle instead.
The extra toughness keeps him going alot longer at the cost of an extra attack.
More than worth it.
I know its a wall of text, but hope it helped out to some degree