The whole point of a plant having a flower, is to attract insects to collect the plants pollen, and therfore reproduce. Over generations certain plants have struck mutual partnerships with certain insects. Therefore every plant is advertising to a different insect to come and collect its pollen, so they each evolve their own coloured flowers to act as advertising boards, and to make themselves recognisable to their particular insect...
I mean what is best flowers
Shifty37 is 100% correct. It's evolution ;) though some of them have been bred or genetically altered by humans to be a certain color or show certain traits.