I would go for an 18v with Lithium-Ion batteries. Any of the top brands are all pretty good - DeWalt, Milwaukee, Hitachi, Bosch, or Makita. Also keep in mind any other cordless tools you might buy in the future. Many brands have an entire lineup that will run on the same batteries and some have entire kits you can buy with several of the most popular tools.
If you are looking for a more high performance professional drill check out the Makita BDF451, Milwaukee 2610-24, or Bosch 37618-01. You can read reviews and compare specs here: http://www.bestcordlessdrillinfo.com/
For a little less money check out the Hitachi DS18DSAL, Makita LXD01CW, or Bosch 36618-02.