I love my Galaxy Tab.
Expandable memory was a must for me, which meant ruling out the iPad and Google Nexus right off the bat. Right now I don't need a ton of space but I don't want to have to buy a whole new device if my use habits change, and the removable card adds flexibility - for long road trips I pop in a card that holds the kids' movies and music, then swap back to my primary card when we get home.
The Nook HD was a close second to the Galaxy when I was shopping for a tablet, but I prefer the "generic" Android OS and direct access to Google Play; when I bought my Tab the Nook was still "locked" to BN's own app store (I think I heard that has changed now?). The new generation can't be rooted easily like the Nook Color could, and I was just more comfortable with a "neutral" manufacturer rather than one marketing the device as a way to sell content. The Kindle was never really much of a consideration for the same reason - I don't like being restricted to just one store or format. On my Galaxy I have the Nook, Kindle, and Kobo apps and buy my e-books from whichever has the best price on the title I want.