From what I've read, Daemons Of Chaos should be good from your specifications.
Daemons can be very competitive, if you play them to their strengths. Some things such as causing fear/hardly ever running away/etc aren't single game winning things, but when all of those little things add up, along with some amazing HQ/etc, they can be amazing. Losing combat can result in some of them vanishing from the game, and some spells can make enemies into allies, so they're fun to play - Coupled with the aggressive nature, they are fun to play.
Magic-wise, they're awesome in my opinion. They have a large variety of spells, some for injuring enemy units, effecting the battlefield, adding dice to the pool or strengthening allies. There are 3 lores of Tzeentch, Nurgle and Slaanesh, that the respective followers can cast if they're wizards. The main wizards - Greater Daemons and Heralds, are also pretty nifty in CC, so they can have several roles. Khorne doesn't use magic of any sort, but with those units you can have some devastating QC fighters. Tzeentch's Pink Horror units count as a wizard each I believe, so you can deal out sorcery in a large variety of ways!!
Horde-wise, it can go either way. Two common setups that I have seen:
Horde-
Large number of core, mixture of Bloodletters, Pink Horrors, Daemonettes and Plaguebearers, coupled with some heavy hitting elites and a good commander. Characters like Epidemus can really shine here, and it's a good play style to run. you will have to paint a lot of models (usually) however they are fun to paint
Elite-
Well, judging from what I've seen you'll favor this. You want some core, or course, I'd say medium sized squads of Plaguebearers and Bloodletters, maybe some horrors as you like magic. Then, you want some more elite things like Juggernauts and seekers. Now, you can have some really awesome units that are good in QC, but are also nice wizards. Maybe a Khorne greater daemon, as a unit up with the main battleline, coupled with a Tzeentch greater Daemon, using a myriad of good magic, both supporting and aggressive.