I am a huge fan of Orks, so am rather biased towards the Orc trilogies that Stan Nicholls has written. The first trilogy contains Bodyguard of Lightning, Legion of Thunder and Warriors of the Tempest; the second consists of Weapons of Magical Destruction and Inferno.
The first three books stand head and shoulders above the others, but that said, I still enjoyed reading the second trilogy. All of the stories are about the Wolverines, a band of Orks that are led by Stryke. The plot follows the Wolverines through various twists and turns in their fortunes, with some great character building and combat scenes and... I won't spoil them for you.
I give the first books 9/10 and the others 7/10, mostly because they are still about Orks.