The "big 19" basic explanation – Espeon (week: 5-1) – A Gengar with better base stats (defense and stamina)
Heracross week: https://pokemongo.gamepress.gg/q-a/big-19-basic-explanation-heracross-week-4-1-only-one-optimal-moveset-good-type-coverages & https://pokemongo.gamepress.gg/q-a/big-19-basic-explanation-heracross-week-4-2-how-about-other-movesets-various-types & https://pokemongo.gamepress.gg/q-a/big-19-basic-explanation-heracross-week-4-3-tyranitar-killer
Espeon, by having the same base attack stat as Gengar, is slightly better because of higher base defense and stamina stat. It is also by far a great Pokémon attacking into Blissey, better than Flareon when only comparing the best cycle DPS moveset.
Flareon has a better base defense stat and the same base stamina stat, but lower base attack stat.
Gengar is inferior to them in most situations against Blissey, ghost hits normal for not very effective damage. Sludge Bomb and Focus Blast are not the best charge move for Gengar. Sucker Punch does a little bit better than Hex here. Also, as stated above, having slightly weaker base defense and stamina stats.
Next post, is still Espeon!