Choose yer poison

The Brazilian wandering spider (Phoneutria spp.) is an aggressive and highly venomous spider regarded by some as the most dangerous spider in the world. The venom of this rather large spider, often found in banana fields is a painful erection that can last over an hour. Scientists are now isolated the component of the venom that mediates this effect and are pursuing it as a potential therapy for penile dysfunction.
Another intersting venom is produced by the Irukandji jellyfish. This small jellyfish provides a sting that is not remarkable... at first... and then turns into hell on earth. After about a thirty minute delay the victim develops "Irukandji syndrome" whose symptoms include severe backache, muscle pain and cramping, chest and abdominal pain, constant vomiting, anxiety (strong wish to dy in many cases), hypertension and pulmonary edema. This last 4-30 hours in most cases but may not clear up for a week.
From Wikipedia:
The severity of pain is apparent in a Discovery Channel show on Carukia barnesi when two researchers (Jamie Seymour and Teresa Carrette) are stung. Even under the "maximum dose of morphine" Teresa remarked that she "wished she could rip her skin off", and is later seen writhing uncontrollably from the pain, while lying on her hospital bed. In a particularly disturbing shot, we see Teresa's feet contorting and digging into the bed. When the camera pulls out to a wide shot, she is rubbing her face, her body is contorting in agony, and her legs are rapidly sliding and kicking around on the bed. Jamie, at his worst, is also seen writhing in pain, curled up like a ball and barely able to speak. Jamie said he wished that he was stung by Chironex fleckeri instead since "the pain goes away in 20 minutes or you die". Another recent program that aired on the Discovery Channel entitled "Stings, Fangs and Spines" featured a 20 minute spot on Irukandji Syndrome. In the segment, a young Australian woman was stung and developed a severe case of Irukandji syndrome. In a testament to the severity of pain involved, a re-enactment (featuring the actual victim portraying herself) shows her screaming and violently thrashing around on the hospital bed in an almost convulsive state, for the bulk of the segment. She later commented that this unbearable pain lasted for hours, and "I didn't think it was possible for anyone to endure that level of pain without turning into a vegetable".

The insane thing is that in 1964 Dr. Jack Barnes confirmed the cause of this syndrome by capturing a jellyfish and purposely stinging himself, a lifeguard and his 9 year old son (!!). Now that's science!