It was a tie between Mark getting his legs chopped off, eaten, glasses taken away (xD), tied up in a bathroom, then turning into a walker, and Ben falling off a balcony, being impaled by a spike, and then getting shot in the head.
But ultimately, Mark.
I mean, it can't have been that bad if Ben said "I'm okay, it's just my leg" after being impaled. I loved Ben and Mark though. My favourite characters.