It really depends on what matters to *you*.
If price is paramount the Sony 300 and the Jetbook/Aluratech twins are kings of the lowball for now. Next month, Kobo joins the party in the US.
Note that, 5" screens are fine as long as:
1- You're used to reading paperbacks (Or on smartphone or PDAs)
2- You choose a reader with good font controls
Now, things you should decide *upfront* if you care about them a lot, a little, or not at all include:
1- Wireless access for ebook purchases
2- touch screen
3- multiple legacy format support (prc, ereader, fb2, lit, docreader, etc)
4- text-to-speech
5- typographical controls (you get to choose any Truetype font, fonts size, line spacing, margins, alignment, hyphenation, etc)
6- SD Card expandable storage
7- motion sensor for screen rotation and other uses
8- Folder-based or flatfile database-driven bookshelf; choose one or the other
9- Note taking/annotation
10- dictionary lookup from *inside* an open ebook
11- Warranty length and terms
12- Non-ebook app support (browser, games, etc)
13- Oh, yes; price...
