To power up, you need Candy of the Pokemon you can get when you capture the same Pokemon in Pokemon Go. That means, if the Pokemon is easy to capture, it is also easy to be powered up with Candy.
Each Pokemon has different CP when you capture it. To save spending of Candy and Stardust for power up, finding a Pokemon with high CP is important.
You encounter a wild evolved Pokemon few times. Since such Pokemon do not need Candy and Stardust to evolve, you can save cost to power up.
Vaporeon has high HP and better moves. Additionally Vaporeon is Water-type Pokemon, so it is effective against to Charizard and Arcanine which you see a lot of times in Gym battles.
Although Flareon has fewer HP, it has higher CP. Flareon is effective against to Venusaur and Exeggutor which you encounter many times in Gym battles.
Moves Jolteon uses are all Electric-type and effective to Water-type Pokemon. So, Jolteon can beat Lapras and Goldak which are everyone's favorite.
Many of Pokemon popular in the early stage, such as Pidgeot and Gyarados, are weak against to Electric-type moves. Magneton can use Electric-type moves and has few weaknesses. Additionally, Magneton is easy to power up by Candy because Magnemite is easy to capture during night time
Since Pidgey is easy to find and capture, Pidgeot is easy to evolve from Pidgeotto and Pidgey. However Pidgey has lower max CP, it works only in the early stage.
Since Psyduck is easy to catch, You can stock Candy for powering up easily. Additionally, Golduck is a Pokemon who can use Water- and Psychic-type moves effective against to Poison-type Pokemon such as Venusaur and Golbat.
Arcanine may be hard to evolve from Grolithe because Growlithe is not easy to be found. However, Arcanine has high CP and use Fire-type moves effective to Venusaur and Vileplume.
Tentacruel has fewer weak types and so it is useful for a Gym battle. Additionally Tentacruel is easy to power up because Tentacool is easy to find by the waterside.
