Lapras has 10x the movepool kingdra wishes it had. It gets Thunderbolt, Ice Beam, Hydro Pump, Surf, Sing, Psychic and a bunch of others. It also has water absorb for an ability which is funsies. Lapras also has Dragon Dance, Ice Shard and Waterfall if you feel like going physical.
Vappy is honestly horrible in game. Kingdra is ok but its movepool is horrible
I will make a list of pros and cons of every mon.
Lapras:
Pros- Water Absorb, Wide movepool, really bulky so you wont have to use potions to heal it, water absorb also helps with this, can be caught earlier than the other 2 pokemon listed
Cons- There are better water types imo, like Poliwrath or Cloyster, kinda has moveslot syndrome
Vaporeon:
Pros: Also has water absorb, is also pretty bulky, get it in Goldrenrod city, which is pretty early
Cons: Doesn't get anything really outside of surf/hydro pump/ice beam/return
Kingdra:
Pros: Dragon/Water only has 1 weakness, limited but all inclusive movepool (Dragon + Water gets rid of just about everything)
Cons: Evolves via trade, need to do mt mortar to get dragon scale, Horsea is caught late game, kingdra is frail :(
Of all those pros and cons i think Lapras stands out as a clear cut winner, you can get it early in game and its really got a fantastic movepool.