What is the best ePub reader?

[freeradical]

Zoidberg wrote:
[quote=freeradical]
I do not have an iPad, and my phone is a 4s, so this will be for reading on a real computer.;-)

I do have an old 3rd gen b&w Kindle, which I rarely use anymore.

So I guess it's Calibre.

The desktop version of iBooks also handles ePub. I've read a number of books on my MacBook Pro while it looks like I'm doing research.
Can I use this app the normal way, by associating filename extensions to be opened by it, or do I have to bring books into the app in a convoluted manner such as with iTunes etc.?