I've only ever read kindle books purchased from Amazon or borrowed library ebooks. But I won an ebook in a giveaway & I'm unsure if epub or doc format would be best on my kindle fire hd? Thanks in advance!

If it comes to you as a word doc, save it as a pdf (if you have MS Office it is easy to do this) and then send it to your account via email with CONVERT in the subject line.

If you get an epub - download the Calibre software and convert it to mobi format.

Neither!    Send it to you amazon account on the Fire & have amazom reformate it!   There will be a small charge but Freebie worth it. 

epub will be formatted more like a book than a doc will.  epub will also be easier/cleaner if you want to convert it to mobi.

i only have a Kindle, not a Kindle Fire, but i'm pretty sure you can download an app to read epub files on your Fire.  Alkido, i think?

you can send it for free to your Amazon account to be converted.  you just use @free.kindle.com with your account name before the @ sign