Hm...The rarest animal I have seen out. I´ve probably seen a lot of birds and insects that are rare but, due to my uneducated mind on those areas, I would´t know.
I was once a private chauffeur for a Natrix Natrix. I came upon it just before it was supposed lose it´s head to a paranoid man with a shovel, and brought it with me in the car to be released in a safe area. I don´t see them very often anymore, unfortunately. I often see the Vipera Berus, but not the Natrix Natrix.
Other than a lizard once every 10 years, It would have to be the cheetah I saw on Masai Mara a few years ago:
Resting Cheetah by
Lykketrollet, on Flickr