Translation:This is the longest snake that I have seen.
My impression is like that: "jakiego" works better here because then it suggests "I have ever seen".
"którego" (it is accepted, by the way) sounds to me like I've been looking at several snakes and this one is the longest I have seen here.
Can we have 'that I have ever seen' added to answers? It is the most natural in English - or is there something very different in Polish that would render 'that I have ever seen'?
You could render the "ever" part by "kiedykolwiek" or "w życiu" (in my life), but this is actually already an accepted answer.
I don't know the exact rule but what think is that: superlative: „naj-” goes only with „jakiego/jaką/jakie/jakich/...".
I think that using "którego" is a shortcut from "To jest najdłuższy wąż, z tych które widziałem" which means "This is the longest snake from those i have seen"
I'm not sure that the English translation heeds the word "that". Not a problem, just an observation.