If your principal criterion is speed of growth, then you should probably avoid the Sativa type strains altogether and opt for the Indica types, which mature more quickly. To get to harvest quickest, you're best advised to grow indoors with the best lighting setup you can afford, using a hydroponic growing medium. The hydroponic method gives you the most precise control of nutrient additives, which can be used to speed up the growth of your plants.
You can also use lighting phases to accelerate growth but exercise caution. If you push your plants too hard, you could end up with straggling monsters of low potency. Once you've got your growing regime right, the most important factor in getting a crop to maturity quickly is your choice of strain.