Tepig is the best starter for a few reasons. Pignite is one of the few Fighting types you can obtain before Lenora, which makes fighting her much easier. In addition, Fire types are fairly uncommon, so being able to guarantee one will do you well into Burgh and Brycen. Emboar's high Attack is useful in conjunction with moves like Arm Thrust and Heat Crash, and though it doesn't get many physical STAB moves, its Special Attack is able to make use of Flamethrower.
It's not without its downsides, though -- it matches up poorly into most of the Elite 4 (barring Grimsley), and its mediocre Speed may hinder its viability. However, it's still the best starter to choose because there are plenty of Water types you can choose instead of Samurott, and Serperior's pure Grass typing and offenses are unhelpful in a Nuzlocke.
An argument could be made for Samurott if you value its consistency as one of the few Water types you get before obtaining Surf. In addition, Darmanitan is probably better than Emboar, so Samurott would be the starter to pick if you could somehow guarantee to obtain Darumaka and don't like any of the other Water types.
