I think the best army would be a mix between GK and the IG infantry army. Been thinking it through and here's what I would use if winning was the goal.
Grandmaster & Termies
2xStormtroopers (Plasma Guns)
IG Platoons with a mix of HW, Flamers & Plasma Guns.
GK Teleport squads
Sentinels w/Heavy Flamers
Orbital Strike- Melta Bomb
Remember the Melta Bomb is a killer so deploy your GKs wisely. The Termies are safe from the Melta Bomb so use them to tie up valuable units in the Melta Bomb blast. They kill other Termies too and the Grandmaster provides you with that one useful force weapon
. Use the teleporting squads to hit intercept units that move towards your army.
I like Missile Launchers myself. Nothing like pinning the enemy near the Orbital Strike area. If you're really nasty get an Inquisitor and Callidus. Use Word in your Ear to move enemy Ordnance out in the open or enemy HW squads out of LOS.
Sentinels are really good with the Heavy Flamer in case you play maps with lots of cover or LOS obstructions.