I like ALreader
ALreader reads HTML, EPub, RTF, FB2, DOC and TXT formats along with Palm formats PDB and PRC. I don't think they've added dictionary support in English yet but it's not a feature I used so I may be wrong. It doesn't support PDF but I never found a suitable PDF reader for my wince device (Wits A81) though the Foxit reader wasn't too bad. It does load the book right where you left off which is nice.
Now that the A81 is my wife's toy she only uses ALreader since it's simple and feature-rich. It does some things that none of the iPad reader apps can match such as change backlighting and text size with a finger stroke and variations in font sizes that neither my Sony 600 or iPad can match. User profiles make it a snap to switch between fonts, sizes, colors, etc. I'm starting to miss using it as it's superior in some ways to anything on my other devices.