High Elves are really fucking high on the tier list for 8th due to how Always Strikes First works, sure they're still fragile but they even get to hit first with great weapons, and the changes to magic benefit them nicely too.
Skaven are the other high end one, but its a LOT of models to paint. You need numbers with Skaven - you get that and the army really starts to steamroll things. The downside is low toughness/leadership and a tendancy to lose lots of guys per turn, but any Skaven army worth its salt will keep taking those losses and just stroll onwards. Plus Hellpits and Doomwheels are just nasty.
With Vampire Counts, you can currently make a ridiculous powerful unit of Blood Knights with a tooled up Vampire Count and a specific BSB build. However, skeletons really don't last long, theres very little in the way of long range offence, Varghulfs are (in my opinion anyway) almost useless now. The meat of the army is in Vampires, and Ghouls can be pretty awesome in horde with poisoned attacks.
Brettonians, haven't played them so I can't comment, but with the way cavalry works now they'd be on a lower tier than most things.
Dark Elves are like a reckless High Elf army. You can get some nice magic off due to the ignoring 6 power dice limit thing, but your spellcasters usually die (albeit in amusing ways). Cold One Knights (imo) hit harder than Dragon Princes, but the drawback is Stupidity. I personally prefer the Dark Elf long range stuff, i've seen it do horrific things, Hydras are nice but now suffer horribly to cannon fire.
Out of all the ones you've suggested, and bearing in mind you have WoC already, i'd say go with either of the Elves. Both have strengths and weaknesses, i'd say High Elves are the stronger due to ASF though.