Blastoise.
Here's a stat check:
Tentacruel:
HP: 80
Defense: 65
Special Defense: 120
Speed: 100
Blastoise:
HP: 79
Defense: 100
Special Defense: 105
Speed: 78
Thanks to superior defense and mmore well-rounded defensive stats, it is easy to see why.
Note: Tentacruel has access to Poison moves, to counter Fairies, so you might want to consider it
Note 2: Mega-Blastoise has excellent offensive power, a great ability, and access to moves to exploit the ability.
So, it can hit Fairies just as hard, albeit without the Super-Effectiveness.
Here are two common Blastoise Movesets:
Blastoise (F) @ Leftovers
252 HP/ 252 (Special) Defense/ 4 Special Attack
- Scald
- Rapid Spin
- Toxic
- Protect/ Stealth Rock
or
Blastoise (M) @ Blastoistinite
252 Special Attack/ 240 HP/ 16 Special Defense
Modest Nature
- Aura Sphere
- Water Pulse
- Dark Pulse/ Dragon Pulse / Ice Beam
- Rapid Spin
[ Credits to &Psychic x for the EV Spread for the 2nd set]