Historically, the first fictions written by fans that I read belonged to the Yami no Matsuei (Descendants of Darkness) universe. It simply happened to be the first anime that I watched :) Following that, I was ensnared by Rurouni Kenshin.
With some series I've not even seen the original but happen to like the fanworks.
Currently I'm into Harry Potter fandom - and have been for some time.
I also enjoy Star Wars fiction with Luke-Vader interaction.
Naruto ficverse is bountiful and full of cool time-travel and power-up fics and I do believe I do not have to introduce Bleach, I'm especially fond of fics about Ichigo's interaction with his hollow.
Stargate-SG1 and BtVS make for great crossover fandoms.
The Hikaru no Go universe is maybe less well known but the overall quality of the stories is that much higher and I enjoy exploring it thoroughly.
Death Note on the other hand is exceedingly popular, so there is a lot to choose from and I have to admit, I'm picky :D Only the best of L and Light fics for me.
And last but not least, I'm addicted to reading great Highlander fanfiction.
My favourite pairings include ItaNaru and HP/LV and favourite character is Methos.
Now for some statistics:
According to FFnet, this profile has been visited from 50 different countries during the last year.
Also, I've submitted over 5000 signed reviews :)