I've always thought Egon was the most accident prone. (*egon) He's been turned intop a werechicken, a baby, a trampoline. He has a pet dragon, and let's not forget he was turned into a ghost! (*egon) In fanfiction, Sheila Paulson has him being horribly injured in A Denethor Solution, has him concussed in A Child of Time, a tag to Who Comes to Lupusville, has him stabbed by a demon in Green-Eyed Monster, and has him suffer pneumonia from being captured by a demon in Home. Gryph or Macx has him being shot in A Bullet With No Name, and has him being concussed in Haunted House. (*egon) So yeah, in fanfic, and cartoon canon, Egon is definitely the most accident prone. (*egon)

I have yet to see all the RGB episodes so I can't judge really.

As far as the movies go I would have to say Ray, he's more mentally accident prone/makes bad decisions than physical though. In the first movie he accidentally decides the form of Gozer, he also decides to attack the ghost at the beginning in the library, which lead to them running away.

In the second movie he gets in the way of the painting at the end, almost makes the slime react a bit too much when he and Egon are creating a negative reaction to it, he accidentally throws the slime all over one of the customers in the restaurant where Peter and Dana are having dinner.

Is not thinking some things through (gee, kinda like me) really the same as accident-prone though?