I'd say the anaconda, merely because the rumor of super huge Pythons is just that - a rumor. I've seen some pretty big ones but the ones of anaconda size seem to be camera shy (like ghosts and aliens and bigfoot). Thus I give the nod to the anaconda.
Edit: I guess I was wrong about there not being a record of the super long python, as they are supposed to be the longest snakes in the world, HOWEVER the anaconda still win in size due to weight as they are the heaviest snakes in the world. Plenty long + all that advantage in girth...my nod still goes to anaconda 😉
Although this anaconda would definitely kick some @$$
"One notable account was reported by adventurer Percy Fawcett. In 1906, Fawcett wrote that he had shot and wounded an anaconda in South America; he reported the snake measured some 18.9 meters (62 feet) from nose to tail."
^ wikipedia, although might not be true, again another super gihugant snake that avoids respectable camera/video evidence, although 1906 isn't a shocker we don't get such evidence, and if it was that huge it wouldn't be easy bringing home any evidence.