epub for my Kobo. I have a few books I could only find in PDF (especially image heavy ones) and I use those on my iPad.

Electronic Publication for the win!

[stillnoar]

Epub

Because they are relatively easy to create and edit yourself.
And if you are a hair-shirt kind of person, you can do it all with just a simple text editor and an archiving program.

Epub. Works on my Fire tablet with Aldiko.

[pony123]

epub and mobi

epub

Mine used to be PDF, but then i got kindle. I love reading books on kindle, so the .mobi format.

[proudbooky]

e-pub for sure! and I hate PDF files

Epub

[agnesm]

Epub

Epub. Because I can upload them to Google books. I have 1012 books on Google books. Mostly star trek, star wars, Terry goodkind, Simon green, Ronda Rousey, and others

I'm sorry I'll ask a very stupid question: why get a Kindle when there is Kobo?
That's why I got a Kobo in the first place, it can read a lot of formats (even cbr/cbz). But then maybe Kobo is more popular here in France anyway.
And of course my favorite format is epub, you can easily change the layout, but also use dictionaries. The only problem is the sometimes awful conversion errors, so sometimes I just stick to scanned books in pdf (even if it takes 3 hours to turn a page).

Epub

EPUB obviously.

EPUB! They're readable on all my devices and available from all sellers save Amazon. Even when I get MOBIs and PDFs, I convert them to epub for ease of use. Though I have a Kindle (or two), I prefer my Nook(s).