That depends on your definition of “snake.” Before I continue, yes, serperior is a good cuddler, but there are others that might be better suited to the task.
For example, arbok is notoriously a good cuddler, being an expert at the technique Wrap. Many consider it to be better than serperior, if only because serperior’s fighting techniques don’t quite rely on immobilizing an enemy that way, which means that you are, quite literally, training arbok to cuddle, more so than serperior. Ekans is just as good as its evolution, but arbok really has the advantage of length (and thus, more length with which to cuddle). Meanwhile, if we expand our definition of “snake” to include all serpents, including those that are more dragons, dragonair can not only cuddle but also change the weather of the immediate surroundings to a rather pleasant temperature for maximum comfort of both its trainer and itself.
Of course, if we expand our definition of “cuddler” to include snakes that are excellent for cuddling, you really can’t get any better than dunsparce.