Tentacruel has been fighting in the upper tiers for a long time for a good reason: a rare and nice typing, a good movepool and good stats.
Set Name: Support Jellyfish
Ability: Rain dish
Item: Leftovers
Nature: Bold
EVs: 252 HP, 240 Def, 16 Spe
-Scald/Surf
-Toxic Spikes
-Rapid Spin
-Protect/Ice Beam
Tentacruel has nice defences, Rain dish, Toxic Spikes and Rapid Spin, so it can support his team in many ways. Scald is usually the best option on a bulky water, but many opponents will be poisoned by the Toxic Spikes, so you may just use the more powerful Surf as those opponents won't get burnt. For the last moveslot Protect works well with both Toxic Spikes and Leftovers/Rain Dish, while Ice Beam adds coverage.
Max HP is necessary to maximize both your defences, 240 EVs in defence with Bold nature to take well hits from both the sides of the spectrum, 16 EVs in speed allow you to outspeed neutral 70 base speed like Breelom and Metagross.
Other Options
Personally I think that those moves are good enough. Not many other options to use...
Partners
Drizzle Politoed or Rain Dance users to activate your ability. Ferrothorn has a scary good sinergy with rain and Tentacruel. Poison types can be annoying as they absorb your Toxic Spikes, so you can use a ground or a psychic type.
Counters
Jellicent blocks your Rapid Spin and is immune to your water move, but it hates Toxic Spikes. Electric pokemon can hit you very hard with a Thunder(bolt). Grass/Poison types absorb your Toxic Spikes and are not threatened by your water attack. Reuniclus doesn't care about poison and can set up on you or just whipe you out with a psychic attack.
Set Name: Specially Offensive Cruel
Ability: Rain dish/Liquid Ooze
Item: Life Orb/Choice Specs
Nature: Timid/Modest
EVs: 4 HP, 252 SpAtk, 252 Spe
-Hydro Pump
-Ice Beam
-Giga Drain/Toxic Spikes
-Rapid Spin/Toxic Spikes/Hidden Power [Fire]
This is an offensive set. Seems outclessed Starmie, but Tentacruel has some different resists, a better special defence, Rain Dish for passive recovery and absorbs Toxic Spikes when comes into play. Hydro Pump is the preffered option as 80 base SpAtk isn't a lot. Ice Beam is obbligatory useful coverage. Giga Drain covers water types, while T-Spikes and Rapid Spin can be used as you will force some switches. Hidden Power [Fire] is recomended outside of the rain as without it the thorny steel/grass thing owns you.
Other Options
You may use a poison move for STAB, expecially if you're using Choice Specs in the rain. Sludge Wave (Bomb) are powerful, but they give you pityful coverage.
Partners
Same stuff as before.
Counters
Pink fat blobs. Special walls in general. Jellicent blocks Rapid Spin too. Thundurus, Jolteon, Alakazam all check you. Dugtrio traps and kills you.
Set Name: A jellyfish playing with swords???
Ability: Rain dish/Liquid Ooze
Item: Life Orb
Nature: Jolly/Adamant
EVs: 4 HP, 252 Atk, 252 Spe
-Swords Dance
-Waterfall
-Poison Jab
-Payback/Hidden Power [Fire]/Substitute/Rapid Spin
Phisical sweeper set. Swords Dance doubles you attack and makes you powerful. Your water and poison STAB surprisingly get good coverage together. The last move is a filler. Payback is your best weapon against Jellycent, while Hidden Power [Fire] roasts your thorny friend when used outside of the rain. Substitute is a good move. Rapid Spin if you feel like you are an Excadrill.
Other Options
Qwilfish. Same moves, more attack, less speed, Swift Swim, Explosion, less bulk, no Rain Dish. It's difficult to choose which one to use. You can use a Baloon and become the jellyfishy version of Excadrill, but you will probably just fail...
Partners
Same stuff as before.
Counters
The bane of all water types: the thorny Steel/Grass thing. Phisical walls not weak to one of your STABs. Jellicent, Skarmory. Thundurus, Jolteon, Alakazam all check you. Dugtrio traps and kills you.