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Espeon - very fast and powerful. Psychic attack everything. Light Screen/Reflect/Morning Sun/Calm Mind are good filler moves.
Umbreon - durable, but can't do much other than help catch shadows/stall. You want to Confuse Ray/Bite everything, eventually teach it Moonlight for best use. Toxic is a good filler move.
Makuhita - good defense and good attack, but Cross Chop with low PP is your only good move. Teach it Focus Punch and Earthquake so it can do more stuff.
Misdreavous - good early on and levitates. Confuse Ray/Pain Split and two attacks is what you want. It doesn't get better and struggles to survive later on.
Skiploom - fast and reliable support. Sleep Powder/Synthesis are too good, replace Mega Drain with Giga Drain.
Flaffy - high special attack and can take hits. It has Thunderbolt and Thunder Wave, a powerful combination. Teach it Protect to defend from Earthquakes, Light Screen is also good filler.
Quagsire - way too op. Reset for Water Absorb ability and Surf/Yawn/Mud Shot then later Earthquake takes care of everything.
Stantler - pretty rounded out good stats and has Intimidate. Confuse Ray/Return everything and Hypnosis/Shadow Ball/Iron Tail is pretty useful.
Vibrava - good typing and stats. Dragonbreath/Earthquake everything, teach your partner Protect.
Everything else has bad moves, can't take hits, or you get it too late. Octillery, Mantine, Medicham, and Altaria are borderline usable but they require things like Ice Beam which is bullshit to get. The legendary dogs are good of course but make the game too easy. Ditch all the starters - Typhlosion only has one good move (teach it Protect), Feraligatr is bad before the Physical/Special split, Meganium can't do damage and Jumpluff is superior support.