The Evolutions morphs are essential as others have said, but if you bought the Genesis Starter Bundle, you should have them already. They should allow you to change waist size, leg length, etc. There’s another very nice general morph set available for free: http://www.sharecg.com/v/58198/
For realistic men and women: I’d start with Michael 5 (get the Pro Bundle if you can—Michael’s clothing and hair tends to look good on the girls, too) and either Victoria 5 or Stephanie 5 for women…V5 is bustier (especially with the included Supermodel shape and breast morphs) and S5 is more petite. S5 doesn’t come with the extra breast shaping morphs, but she has three face shapes and three body shapes to Vicky’s one face and two bodies. Take a look at the Ethnicity bundle also (or SickleYield’s Diversity and similar sets, sold at Renderosity).
If you want to do nude renders, note that Genesis genitals are only available in certain bundles:
Michael 5 Pro Bundle (male)
Michael 5 Pro Suite (male)
Victoria 5 Pro Bundle (female)
Victoria 5 Pro Suite (female)
Stephanie 5 Pro Bundle (female)
Genesis Michael Victoria Combo Bundle (both male and female)
For anime: Hitomi and Hiro 5, and their accessories.
For creatures: Genesis Creature Creator Bundle, Epic Wings, Gorilla, Troll (you should have the Troll already as it came in the Starter Bundle), Tamesis Bundle, and look at various creatures by RawArt and Valandar, among others.
In general: The Gen 3 Iconic Shapes Bundle is great, especially if you have a lot of clothing for third-generation figures like Michael 3, Aiko 3, etc. David 3 for Genesis is sold separately, but unlike the others he comes with extra head shapes which is nice. The V4 and M4 Shapes for Genesis are required for a number of characters in the store, but they are less essential now since the release of V5 and M5. Still, the more shapes and morphs you have, the better! And skin textures, too, for that matter…if you have some built for Victoria 4 or Michael 4, they will work fine on Genesis too.
One more important note: if you are buying a bunch of stuff, join the Platinum Club ($30 for one month) and get 30% off all DAZ Originals. Should end up being cheaper that way…most of the items I mentioned are DAZ Originals.