Stormcast: slow,low model count, shock and awe based gameplay that synergizes incredibly well with itself. Hits like a train if you roll 6's
Bloodbound: buff.buff.buff. murder everyone. Your foot soldiers (chaos/blood warriors) are damn good and are made better by the cheap buffs you can vomit. Even your cheap reavers can dump bucket loads of dice at your opponent.
Slaves to darkness: basically whatever flavor of chaos you want
Ever chosen: if you have the cash to field an army of archaeon and varanguard...I've got a wishlist you could fulfill with the money you save buying an army with more than 3 models
Nurgle/pestilens: Nurgle is heavy survivability, slow not overwhelming in combat, but nigh unkillable. Pestilens are the opposite, super high damage potential with no survivability. All pestilens have the burger keyword.
Slaanesh: SPEED! Turn 1 charges? Yep. Winning combat if you charge ? Sure. Capping objectives at war speed? You got it boss
Sylvaneth: ok, this one I don't really know. Your big units are phenomenal (except drycha who's great in some cases) but your battleline sucks ass. You can teleport every turn
Dwarves: exactly what you think, they're slow, heavily armored, stout little bastardy with gnarly tech. Think gimli with guns
Fyreslayers: angry.fucking.dwarves. minus the guns. Hard to use solo, strong with other armies.
Ironjawz: cool models, unfortunately you have like 6 units. Good news. The models you use are great in combat. Megaboss is an army into himself, gordrakk is horrifying anywhere near anything that isn't seraphon.
Seraphon: incredibly versatile, you can summon Amy unit for any situation, your magic is solid to great and all of your units are decent to stellar. Plus dinos riding dinos. Plays however you want.
Let me know if you want specifics on any race, I know I left out death because I'm not experienced with them.