From all of the reviews I've read and gathered (I've been considering an ereader for over a year now) it really seems to come down to the
Amazon Kindle or the B&N Nook.
Between those 2, though, it really comes down to what you are looking for.
If you
buy a lot of books, then the Kindle will probably more to your liking.
If you are looking for the opportunity to buy
and borrow ebooks from your library, then the Nook is your best option.
To me, Wi-Fi is important because I can buy a book while I'm on vacation. Paying extra for 3G, though, wasn't high on my list, since I have not had good experience with 3G devices in the past. Also, the fact that, anymore, you're typically not too far from a Wi-Fi hotspot anyway.
The bottom line, though, is that people who have bought the Kindle love it, and the people who bought the Nook also love it.