What is your favourite battle of the Horus Heresy? It can be any battle, space, land, psychic and so on. I'll include skirmishes in here also in case there is some particular small action that you particularly enjoy.
My favorite is Istvaan III. It is a classic 40K environment with ruined gothic cathedral buildings and Space Marine on Space Marine action. It also has the drama of a last stand with the loyalists desperately holding out against almost infinite odds.
The battle doesn't have quite the range of scenario potential as other battles do but it still has quite a bit. You can do small scale scouting actions where traitors are trying to find out where loyalist outposts are. You can do ambushes as the Loyalists have taken up defensive positions and are attacking the Traitors as they are coming up.
The ruins and the desperate of the defender's situation create an atmosphere that is easily recognizable on the tabletop.
There are a great number of heroes involved from both sides.
After that I would have to go with 1st Prospero. It has such a sense of tragedy about it. Both sides view themselves as loyal to the Emperor and the Thousand Sons just can't understand what is happening to them.
They have been betrayed by their fellow Legion, they had been betrayed by their Emperor and worst of all they had been betrayed by their Primarch who left them high and dry to be slaughtered by the Imperial forces.
The Thousand Sons and Vlyka Fenryka were also well matched adversaries in that you are pitting the savage ferocity of the one vs. the psychic mental power of the other.
On top of that you have a lot of options with scenery depending on when you are placing your game in the context of the battle. Is it right at the beginning where most of the city is still intact? Then you can put lots of really cool looking buildings on the table.
Is it late in the battle when everything is rubble?
Is it taking place inside one of the pyramids then you have tunnel fighting.
There is a part of Tizca where there are portals and the Thousand Sons used these to launch repeated ambushes against Imperial forces. You could do some great scenarios there with a small force of Thousand Sons pinning down a much larger force of the Rout.
So what is your favourite battle of the Heresy?