I know that Nunu's ult area is invisible if he casts it in a bush and the enemies are outside the bush, but I was wondering if Panth's is ult visible to enemies outside the bush if he is landing in the bush.
No, Pantheon's ult is still visible. The logic here is that Nunu's ult is centered on himself. Thus, if you walk into the bush during a mistimed Nunu ult, you could get lucky and interrupt him. On the other hand, Pantheon's ult is targeted somewhere else on the map. His intended target usually has no way of interrupting him, so hiding the AoE marker would be unfair.