The best fish to breed for profit are any fish you can raise quickly and can sell in quantity OR very highly priced fish that don't spawn often or require a lot of tankspace for growing.
The best way is to find some local shops or wholesalers and ask them what they are willing to buy from you and how frequently they will buy them.
I'm currently setting up a hatchery for Freshwater Angelfish. You can see what I am doing in my thread " Wigglers 2: Birth of a hatchery "
But as jetajockey said, if you don't really like the fish you are breeding, it's very easy to become soured on the experience.
Hope this helps