best activation function for ensemble?
I think you will have to experiment; there isn't generally a one-activation-fits-all for hierarchical models. Give the ones you mentioned a try, but perhaps starting with sigmoid and tanh and also a LeakyReLU.

The reason I'd perhaps leave out a normal ReLU at first is because that too would potential trim out information before the final model, as it simply removes negative weights to zero. A good final model should be able to correct for this, but I think its unnecessary to force it to.