In general GW's model designs have improved a lot over the years. Most new designs are at least as good as the ones they replace or better.
The current skeleton plastic set is much better than the previous one.
The blood knights are very impressive - maybe a few unnecessary spikes but still they look great.
Most new skaven models are also a great improvement over their 6th ed versions (especially clanrats, stormvermin aren't that good but still comparable with the old version)
I agree about the hydra - much better than the previous one.
Also the new mounted marauders are much more impressive than their 6th ed versions.
As for chaos knights - they look nice but personally I prefer the old metal versions - they wore fewer decorations but this gave them a very nice, simple, feral look the new plastic models lack...
Not all GW designs are that great though. I hate the new minotaurs and phoenix guard for example. As for plastic ghouls - I never liked them (small, hunched, look stupid), all 74 ghouls I have are metal models.
Anyway in the link below you'll find the best ghoul models ever (imo), made by heresy miniatures.http://www.heresyminiatures.com/hv020.htm