Arbok vs Serperior
Okay, I'm not even going to bother explaining why I'm pitting these two together. Let's just jump right to it, shall we?
In a battle, Arbok has a clear type advantage and with Jaroda's horrid movepool, the Gen I snake is clearly the most likely to win.
In general however, Jaroda is more versatile than Arbok and has better options (Antagonist, Glare, Leech Seed) so you can get more mileage out of Jaroda than you can out of Arbok.
Who do you prefer?
Who'd win in a fight?
Who's more useful?
I'm going with Jaroda (GASP!) because of his awesome design and Grass typing. Also, the few options Jaroda does have at it's disposal are really excellent options. Arbok, on the other hand, is quite one-dimensional.