Mantine has 140 base Sp.Defense? Too bad it sucks othewise.
Politoed and Walrein were great last gen, but the lack of perma-weather hurts those 2 a bit. Ludicolo's a bit rain dependent as well. Out of the ones on your list, these are probably the best candidates:
Tentacruel, Vaporeon, Milotic, Jellicent, and Rotom-Wash
Sets for each:
-Tentacruel w/Black Sludge
Nature: Timid
Ability: Liquid Ooze
EVs: 248 HP, 244 Defense, 16 Speed
Moves: Scald, Rapid Spin, Sludge Bomb/Wave, Protect/Substitute
-Vaporeon w/Leftovers or Lum Berry
Nature: Bold
Ability: Water Absorb
EVs: 248 HP, 252 Defense, 8 Sp.Defense
Moves: Scald, Wish, Protect, Toxic/Ice Beam/Roar
-Milotic w/Leftovers
Nature: Bold
Ability: Marvel Scale
EVs: 248 HP, 252 Defense, 8 Sp.Attack
Moves: Scald, Recover, Ice Beam, Haze/Dragon Tail
-Jellicent w/Leftovers
Nature: Calm
Ability: Water absorb
EVs: 252 HP, 32 Defense, 220 Sp.Defense
Moves: Scald, Will-o-Wisp/Toxic, Shadow Ball
-Wash Rotom w/Chesto berry
Nature: Bold
Ability: Levitate
EVs: 252 HP, 252 Defense, 4 Speed
Moves: Hydro Pump, Thunderbolt/Volt Switch, Will-o-Wisp, Rest
or
-Wash Rotom w/Leftovers
Nature: Calm
Ability: Levitate
EVs: 248 HP, 32 Sp.Attack, 220 Sp.Defense
Moves: Hydro Pump, Volt Switch/Thunderbolt, Will-o-Wisp/Thunder Wave, Pain Spilt
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