Well I will put my 2 cents about 2 of those 3 armies and I'll had a comment about the current army I'm playing because it's by far the best of them all (for me
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Chaos Undivided:
I love Chaos in general and a Chaos Undivided army give you the ability to have all the Chaos powers reunited under the same banner.
The first Chaos army I made was a Chaos Undivided army. With the possibility to have a thousand different configuration I still love to play it once in a while. You can either make it extremely shooty or completely assault oriented. You can have a special squad of all the gods if you make it a Black Legion army list and have swarms of chaos marines advancing on the ennemy with a squad of Berserker of Khorne charging in front of them, a squad of Noise Marines wreaking havok from afar, a squad of infiltrating Plague Marines who are a pain to kill and a squad of Thousand Sons marines slowly advancing while rapid firing their bolters. Put a dreadnought in there wih a predator and you have a full and diverse army to play with. The strenght of this list is it's versatility.
What I love about this army is that you can specialise it toward shooting or close combat depending on your mood and just play it the way you want.
Another strenght of this army that have been disregarded so far is the ability to summon all the Daemon type. They are a truly powerfull addition to the army, my 2 favorites being the Furies and the Daemonettes. Once you tried them you'll never go out without one of those. In squad of 10 for the Furies and of 6 for the Daemonettes, it's crazy how they destroy anything that don't have a power armour and even those who have one don't have much chance against a S5, I5 for the Furies and against the Daemonic Talons of the Daemonettes.
Iron Warriors:
Well they have been discussed before so I will only say what I like best about them. They are a never ending firing line that almost never move so the only real tactical choice you will have to make is during deployment. The fact that you can have more than one Obliterator squad is enought to take this army. With a good mix of vehicule, heavy support squads and obliterators this army is the most shooty one. The basilik and vindicator are of course really nice and rather cheap for a blast template. Ready! Fire! and enjoy
I see two annoying weakness points for this army and they are the reasons I don't play with it often anymore. The first one is that it's a very static army (it become boring really fast) and even if they have a good amount of fire power they won't be able to outshoot the really shooty army of the W40K world like the Tau or the IG. Also if a true CC army get into CC with them they are generally dead and there I'm thinking about a Tyranid army that can both bring CC power and numbers on the table. They are also looking bad against really fast moving Space Marine army, so you should think about it before starting playing it.
Death Guard:
My current army is a Death Guard army. I know you didn't ask about them but you should think about them, they truly rock.
I realised while playing with them in small squads of 7 in my Black Legion army list that my Plague Marines where a pain to kill with their high toughness. They where almost making back their points worth in killing and where a real staying power in the battlefield. I started to take more squad of them to replace the Noise Marines and the Thousand Sons that I didn't really liked (not my play style) and I finished with 4 squads of Death Guards mixed with some Chaos Undivided squad that provided them with fire support. That army evolved into a Death Guard army really fast when I realised the power of a squad of 14 Death Guards with a free AC.
The fact that they can't have heavy weapons is not really a weakness for me as it gives you the opportunity to make a special weapon army, something I learned to love and that is truly powerful. The plasma guna and the melta guns are two awesome weapons that are often disregarded in favor of the equally powerful missile launcher and the stronger (but more costly) lascannon. The comment I usually get when playing them is: I hate this T5!!!
It is a slow moving army but you can put them in rhinos and you can give them infiltration and the Manreaper Daemon weapon is worth the army all by itself. 65 Death Guards (the usual number I have for a 2000 pts army) with a dreadnought, 2 rhinos and a predator are easyly worth a hundred Tau with 2 hammerhead or a any Space Marine army for that matter. They cut throught other Chaos army fairly well too and you can either use them as a close combat army or a slow but rapid firing army. Remember that they are the only Chaos Space Marines that have True Grit.
So In conclusion they are all a ton of fun and you get to choose wich on you like best.