Feraligatr's worst matchups are Chuck, Pryce, Clair, and Lance, so you'd want a Pokemon that's at least neutral in these 4 boss battles. Because so few Pokemon are useful against Lance (and some of them, like Graveler, are bad in those other boss fights), there aren't too many good options.
Ampharos @ magnet/choice specs
EVs: 100 Spe
- discharge
- charge beam
- thunder wave/focus blast/signal beam
- thunder wave/focus blast/signal beam
Note: delay evolution by just one level to learn discharge earlier
Ambipom @ silk scarf
Abililty: pickup
EVs: 100 Atk
- return/last resort
- dig
- shadow claw
- tickle/last resort
Note: use friend balls for a strong early-game return
Magneton @ magnet/choice specs
EVs: 100 Spe
- discharge
- flash cannon
- charge beam
- thunder wave
Miltank @ silk scarf/wide lens
Ability: scrappy
EVs: 100 Spe
- return
- zen headbutt/earthquake
- defense curl
- rollout
Note: use friend balls for a strong early-game return
source
Of course, if you're okay with using 2 water Pokemon and okay with catching a level 50 wild Pokemon right after defeating Morty, you can use the repel trick to catch the Magikarp on Route 43.
Gyarados @ silk scarf/mystic water
- waterfall
- dragon dance
- strength
- flail/ice fang
Note: use a rare candy immediately after catching it, otherwise it takes a very long time to raise a Magikarp to level 51.