Not surprisingly, the market for Kris Bryant rookie cards is quite hot. The premier base card for Bryant right now is the 2015 Topps #616 Rookie Card (RC). There's also a photo variant version that is currently selling for a couple of dollars less, and the original seems to be the much more desirable version.
The Bowman Chrome and Topps Chrome rookie cards of Kris Bryant are fairly good values right now, as well, although warping issues tend to make these somewhat less desirable. The Topps Chrome Prism Refractor is probably the best overall value as far as rarity is concerned and it would be my pick if you're a serious Rookie Card collector.