Here's a list of Pokemon that currently doesn't evolve. Although they doesn't have any evolution phases available, their relatively high CP will compensate for it. To get one of them with high CP rating, it is best to hatch them from Eggs early game around Lvl 20+, or catch them from the wild once you reach Trainer Lvl 30+.
- Pokemon GO: Pokemon Evolution List - Number of Candies Needed
- Pokemon GO: FAQ, Tips, Tricks and Strategy Guide - Start Smart