Based on the features you listed as important, I would think the Kindle 1 is best for you. There may be other brands I'm unaware of though, I really only researched Sony vs. Kindle.
readability - I think they're all equally readable.
replaceable battery - Available on all except Kindle 2
light to read in the dark - Only on Sony PRS-700. However, the 505 has the light-wedge cover (light it built into the cover) which is very handy. This was one of my big needs too. Kindle has 3rd party lights you can clip on.
built-in dictionary - Kindle 1 or 2.
support for a variety of formats - Well, this is the tricky part. Check out the handy
Matrix. Sony supports the Sony DRM format and ePub. Kindle supports Amazon store and Mobi - NON-DRM. It is possible to convert DRM Mobi to Kindle.
ability to buy other than from amazon - I'm not sure how to answer this one, but I believe the Sony has more options here.
sturdiness - I think they're all pretty sturdy. Popular opinion seems to favor the Sony for usability.
memory card access - All but the Kindle 2.
Having just gone through this decision, I wanted the PRS-505 but ended up with a used Kindle 1 simply because there are a lot of them on Criagslist right now. A lot of people upgraded to the Kindle 2 and I found a still-warrantied version 1 for a good price.
I use a paperback
Light Wedge on the Kindle and it works great. I'd rather have one like the PRS-505 has with the
built-in wedge but oh well. I was even OK with the light wedge on paper books even though you have to move it every time you turn the page. With an e-reader they're perfect.
I'm also able to convert my public library's ebooks (Mobi/PRS) format pretty easily and read them on the K1.
The added benefit to me is that I can still buy wirelessly from Amazon when I want to. I would rarely, if ever, buy a new hardbound book only because there are literally millions of paperbacks I've never read. First-run books for $10 is pretty cool to me.
Oh yes maybe the most important part, the Sony software doesn't work with the Mac - it's PC Only.