I know a lot of people have been using Anakim transforms quite successfully at Castle sieges. Was just looking at a list of transforms, and looks like Venom would be a good one too if you have great magic attack already.
Venom Sonic Storm
Attack the surrounding area by detonating the sword energy. Power 2957.
And an Atk power boost
Benom Disillusion
Temporarilly increase your attack power. Effect 1.
This skill of venom is around yourself and not like anakims which is ranged aoe so i can bet that a warlord could easily outdmg and have stunned ppl too.
I think after anakim it will be the vail master transf. 1) cause i see many buying it and heros shouting to buy it. and 2) cause veil has 2 skills 1 single target with low reuse and is equivelant to prominance +20+ (it has power 120 or 126 i cant remember exactly) and 2nd skill is an aoe with the same power. Also has cleance!