The Deathwing Knights and Belial are a good combo, but if they didn't annihilate their target on the first turn of the combat, they generally ended up stuck in, or smoked. They are a bit pricey for a tarpit, so I'm going to try something new for the next one.
I've liked the idea of Black Knights from the very beginning, although I had bad luck with them in the first few games I played. I've got eight of them built right now though, so I'm going to see if I can make it work. I think that in general they can be a more versatile unit than the Deathwing Knights, and can provide some essential maneuverability.
This strategy is going to start with the HQ. I have tried Sammael, and he is good, but he's not great. He gives the unit a plasma cannon, makes them fearless, and provides an AP 2 power weapon, but he's not that hard to kill. Instead of Sammy, I'm going to try a slightly cheaper Interrogator-Chaplain. He has the same 3+/4++ save, but no eternal warrior, which really doesn't matter that much anyway for bikers. He also has a captain stat-line with one point less for his BS, so he can hold his own in close combat. He will be armed with the Mace of Redemption, a S +3, AP 3 power weapon that goes at initiative and has the blind and concussive, and Deathwing weapon (AP 2 against Chaos Marines) special rules. In addition, he has an auspex, to make the Knights plasma talons more effective against opponents in cover. He also gives the unit fearless, and re-rolls on the first turn of combat (not just the turn they assault).
I think in general he's going to add more to the overall effectiveness of the Black Knights, especially once you factor in the benefits of rad grenades. I am planning on running seven right now: a Huntmaster with a melta bomb, five standard Knights, and one with the grenade launcher.
I used some Black Knight bits on a Dark Vengeance Ravenwing sergeant. I really like his robed body, but he is not wearing a helmet and is not really suited for a head-swap. What I've decided to do instead is give him a face-paint war-mask in either white or red. He's adorned with plenty of other skulls. When its all said and done, their shouldn't be any doubt who he's supposed to be.