13.
In gen 1 Nidorino could be evolved into Nidoking almost as soon as he became a Nidorino, giving you a fully evolved strong late game pokemon before your second badge. There's a moonstone right there in Mt. Moon, and after it learns double kick--which is at level 12 in Pokemon Yellow, there's not much else it learns by level up to look forward to in the older games, so you don't run into the issue of it losing access to important moves if you evolve it too early like many stone-based evolutions. Things like earthquake, body slam, horn drill it could always get later by TM.