Any familiar kinds much sold as growing plants that you don't see on seed racks or in seed catalogs would be likely to fall into that group. Part of the time it will be them not tending to come true from seed, and not necessarily that they are hard to grow from seed otherwise.

It would be easier to answer the question if you gave a list of the flowers you want to grow, but in general I'd say to buy the plants that you cannot find seed for, and look to buy plants that are fussy about growing requirements or that have a very long growing season.