If you are focussed on exp via evolutions, then catch the 'low cost' evolution mons, such as pidgey, weedle, wumple, whismur.
These only cost 12 candy per evolution and are pretty common spawns. (maybe Aron too, I forgot the cost)
You also get an exp bonus for adding new mons to your dex, so save up your candies from all your catches, put on a 30 minute lucky egg (2* xp boost) and evolve as much as you can while the bonus is active.
Raiding and higher tier friendships also net more exp - up to 10k exp for a tier 5 raid and 50k exp for an Ultra friend - these numbers are also doubled by lucky eggs...