I would say Jenna's basics and beyond- she's got 3 wonderful DVDs of increasing level. If you have to get only one get Basics n beyond.
I bought mine from Jenna's site:
::: BeyondBellydance ::: Jenna ::: Venture beyond your expectations with bellydancer Jenna's solo performances and group belly dance classes; based in Manhattan, Jenna is available for Middle Eastern dance workshops and private lessons as well.
I also recomment Salome's store.
We got Instructional DVD thread here with lots of reviews:
What instructional DVDs/Videos you have?
For drills I love Ariellah's yoga and belly dance DVD- it is tribal fusion one, the drills really help to work on basic moves!!
good luck.
*hugs*