The best replacement for slaking is snorlax.

The best alternatives to slaking are rated as following:
★★★★★  snorlax
★★★★  regigigas
★★★★  vigoroth
★★★  hariyama
★★★  exploud
★★  aggron
★★  blissey
★★  rampardos
★  zangoose
★  tyrantrum
★  ursaring
★  metagross
★  grovyle
★  breloom
  absol
  vigoroth better

Your alternative for slaking is not listed? Suggest!
Speaking about slaking, one probably shouldn't forget about skill swap.

Other search terms related to slaking and their ratings are:
★★★★★ skill swap
★★★★ hyper beam
You have in mind a companion for slaking and it is not yet here? Propose!



Google Trends

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slaking
snorlax
regigigas