Positions where the bottom has their knees to their chest and your penis is perpendicular to the bottoms hole are your best bet.
Having said that, the prostate is only 2 inches deep roughly behind the base of the bottoms cock from the inside. So positions that allow you to apply firm pressure to that area may be better.
Missionary - or variations of, with the bottoms legs over your shoulders can be good for deeper penetration. I've also found that riding a smaller dick is good, especially if you face towards/away from your top depending on how his dick curves. Spooning positions work for me too, his dick really rubs my prostate then.