Linux
I had only used Linux a few times until I moved to TalkTalk where all development was Linux based and I have been hooked ever since. I love the wide range of choice available and the power provided by the shell. My current preference is for the Mint distro this is because it enjoys the support that Ubuntu has but with everything I dislike about Ubuntu removed. While I would not describe myself as an expert I know enough to be able to perform most tasks with minimal Googling.