Well I think we should define what a "bad trip" is.

I've had uncomfortable experiences on San Pedro, but it was mainly physical reactions.

DMT is NOT even close to the top of the list.  I've been to hell before with DMT, however because of its short duration it is not all that bad.

Mushrooms can lead to a bad place, I've seen it quite a few times and experienced it once.  The long duration mixed with an overpowering experience can lead to an unpleasant experience.

Aya doesn't really give "bad trips" because even the most uncomfortable experience brings rich and powerful life changing healing...however, it can be EXTREMELY overwhelming and uncomfortable at times.

The only one that I cannot fathom having a bad experience on is 2C-I. While it can provide a very rich and rewarding experience with plenty of eye candy it is very gentle on the psyche.  The worst that I have had happen is uncontrolable giggling for 2-3 hours. 

2C-E can be a bit much for some people esp at doses above 20mgs.