Python style

http://www.dragonslist.com/threads/11260-Question-on-Shaolin-Snake-kung-fu
Python is the Snake style that uses leopard fist, for pinpoint strikes and for grappling purposes. As for which Snake style has the most complex forms, they all are complex, each one starts off with simple forms that progress in difficulty as your skill advances, just depends on what type of complexity as each has different characteristics. The most complex of any Snake style are the forms that combine all three Snakes into one, which requires you to be loose, flexible, and be able to make rapid changes in body and mind, as you might use a Snake technique of one type and then switch to another Snake technique and so have to change your focus, positioning, intent, purpose, and technique style with the speed of lightning. As for which I would rank in order of complexity; 1) Viper, 2) Cobra, 3) Python, but those are just my opinions, another practitioner might give you a different order depending on their body/movement type and personal preferences.

I'll respond to Infrazael first; Viper uses techniques that are a little more "showy" and the theory behind Viper is to inflict serious strikes to veins and arteries, thus effecting the blood flow to the brain, and other areas, and also cause psychological damage from those strikes as well. It tends to use Snake Tongue strikes ( Two-finger strikes). Cobra uses less "showy" techniques, and the theory behind Cobra is to inflict heavy damage to the nervous system by pressure point strikes, a.k.a. Dim-Mak, it tends to use the Snake Head strike, hand bent 90 degrees to the wrist, thumb curled in, and gets straight to the "point" when attacking.
Now, Hum Sop Lao; As for complexity of Snake schools, I would say it is a tie between Emei and Shaolin, both take a long time to learn properly, and get into highly advanced techniques and forms. As for which school dominates in Snake, the three I know/practice are Emei, Shaolin, and Wudan, I would have to say Emei is the best and most effective concerning Snake, though Shaolin is a very close second.
You can get videos for Emei/Wudan Snake from Kungfu Qigong magazine, or their online store, the Emei form is done by Helen Liang and is almost identical to one of our Mei Hua Snake forms, White Headed Serpent, and for video on Shaolin Snake, you can try ESPY TV, I think their site is espy.com, their tapes are done by Rondie Chen, a member of the Beijing Wushu Team, and are pretty good examples of the difficulty of the Shaolin Snake system.
Python is first and foremost a Snake system, but does use grappling techniques in its curriculum. It uses its grappling techniques to lock on to an opponent and either break bones or suffocate by asphyxiation, just as a python does to its prey. Most Snake systems have some type of takedown, but their usage is different than the Jitsu systems use, so to compare between the two styles I would say Jitsu has more broad ranged grappling skills while Python focuses its skills upon the movement of the Snake and how the snake would use them; such as wrapping around and crushing, breaking, suffocating. Hope that clarifies.