That's the proverbial “crystal ball” question, much like “what stock should I buy?”

The answer will start with “It depends.” Options to consider: growing in container or in the ground, marketing locally or distantly, wholesale or retail. If you want to market locally in a retail setting you should have some variety of species, some flowering and some conifer. A variety of pricing, species and sizes should be considered. For wholesale: you'd probably want to start with a few species in a selected size range. Think about how you'd market those plants and how you'd get them to market. Once you have some experience some expansion may make sense, or just as likely, narrowing of focus for a specialty may work out. In any event the above issues need some thought first. The good news: bare root seedlings and plugs are the least expensive way to get started.