Lapras, 'cause it's ice type would be good against dragons.
Lapras and other Water/Ice Pokemon are terrible for bulky offensive roles because of their common weaknesses to Grass, Electric, Rock and Fighting. Nearly every Water Pokemon learns Ice Beam, so there's no need for the secondary Ice type to hinder its defensive capabilities. Other bulky Water Pokemon to use are Vaporeon, Jellicent, Clawitzer, Tentacruel, Mega Blastoise, (Mega) Gyarados, Gastrodon and Azumarill:
Vaporeon:
-Wish
-Protect
-Scald
-Ice Beam/Roar/Toxic
Nature: Bold
EVs: 248 HP/252 Def/8 SDef
Item: Leftovers
Ability: Water Absorb
Jellicent:
-Scald/Shadow Ball/Night Shade
-Will-o-Wisp/Toxic
-Recover
-Taunt
Nature: Bold
EVs: 248 HP/252 Def/8 SDef
Item: Leftovers
or
-Scald
-Shadow Ball/Ice Beam
-Will-o-Wisp
-Recover
Nature: Calm
EVs: 248 HP/36 Def/224 SDef
Item: Leftovers
Ability: Water Absorb
or
-Water Spout
-Hydro Pump/Surf/Scald
-Shadow Ball
-Ice Beam/Giga Drain
Nature: Modets
EVs: 172 HP/252 SAtk/84 Spe
Item: Choice Specs
Ability: Water Absorb
Clawitzer:
-Water Pulse
-Aura Sphere
-Ice Beam/Dragon Pulse
-Dark Pulse/U-turn
Nature: Modest
EVs: 164 HP/252 SAtk/92 Spe
Item: Life Orb/Choice Specs
Tentacruel:
-Scald
-Sludge Bomb
-Rapid Spin
-Toxic/Knock Off
Nature: Bold
EVs: 252 HP/224 Def/32 Spe
Item: Black Sludge
Ability: Liquid Ooze
Mega Blastoise:
-Water Pulse/Scald
-Dark Pulse
-Aura Sphere/Ice Beam
-Rapid Spin
Nature: Modest
EVs: 252 HP/252 SAtk/4 SDef
Item: Blastoisinite
Gyarados:
-Dragon Dance
-Waterfall
-Earthquake
-Bite/Stone Edge/Substitute
Nature: Adamant
EVs: 32 HP/252 Atk/224 Spe
Item: Gyaradosite
Ability: Intimidate
or
-Substitute/Taunt
-Dragon Dance
-Waterfall
-Bounce/Earthquake
Nature: Adamant
EVs: 120 HP/64 Atk/100 Def/224 Spe
Item: Leftovers
Ability: Intimidate
or
-Rest
-Sleep Talk
-Waterfall
-Roar/Dragon Tail
Nature: Impish
EVs: 248 HP/252 Def/8 SDef
Item: Leftovers
Ability: Intimidate
Gastrodon
-Scald
-Earthquake/Earth Power
-Ice Beam/Toxic
-Recover
Nature: Relaxed/Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
Ability: Storm Drain
Azumarill:
-Waterfall
-Aqua Jet
-Play Rough
-Superpower
Nature: Adamant
EVs: 212 HP/252 Atk/44 Spe
Item: Choice Band
Ability: Huge Power
or
-Belly Drum
-Aqua Jet
-Play Rough
-Waterfall/Superpower
Nature: Adamant
EVs: 92 HP/252 Atk/164 Spe
Item: Sitrus Berry
Ability: Huge Power
or
-Aqua Jet
-Play Rough
-Power-Up Punch
-Waterfall/Knock Off
Nature: Adamant
EVs: 16 HP/252 Atk/240 SDef
Item: Assault Vest
Ability: Huge Power