For myself, I use the iBooks reader on my iPad. But I should say, I don't buy a lot of ebooks myself. I oversee the ebook production where I work. And iBooks is still one of the best as far as supporting the full epub standard. Kindle has gotten better at that lately, though, I have to admit, with better support for MathML and SVG and better table handling. It used to be trying to view tables on the Kindle sucked ass, but it's not so bad now. I have limited experience with Kobo devices.
