Here's the set I use:
Item: Leftovers/Black Sludge
Ability: Liquid Ooze/Rain Dish
Nature: Calm (-Att, +SpD)
EVs: 252 HP, 4 Def, 252 SpD
-Ice Beam/Giga Drain/Knock Off/Protect/Acid Spray
This set maxes out Tentacruel's special walling and support abilities. Leftovers and Black Sludge serve the same purpose; Black Sludge hurts anything that wants to Trick you, but could also hurt one of your teammates if it gets Tricked to them. Liquid Ooze should be used if you lack Politoed's infinite rain, as it allows Tentacruel to take on Ferrothorn with far more confidence than it deserves, as Liquid Ooze makes Ferrothorn take damage from the Leech Seed it likes to spam, and Scald's Burn will cut damage from Power Whip in half. Scald is the obigatory STAB move that has a chance to burn. Toxic Spikes is an entry hazard that gets low distribution, and is thus highly reccomended on this set. Rapid Spin keeps annoying hazards off your side of the field. Ice Beam is the primary option, as it gets great coverage with Scald, as well as fending off Dragons such as Dragonite and Salamence. Giga Drain can be used to beat off the increaingly popular Gastrodon, as well as the lesser used Swampert and Quagsire. Knock Off is a great utility move, as every Pokemon in the game relies on their item to some extent. Chansey?
Without Eviolite, this blob ain't walling anything. Hydreigon? Far less threatening without that pair of Specs. Haxorus? Without that Choice Band, it hits like a leaf slowly drifting down from a tree in fall. Actually, no, it still hits like an 18-wheeler at highway speeds, but now it isn't going at 300 Kph. Protect can be used with Rain Dish and Politoed support to recover an additional 12.5% a turn. Acid Spray is a bit of a gimmick move; it's essentially a 40 BP Metal Sound with 100 Accuracy, which can be used either to support a sweep or to increase damage from Scald, or even Acid spray itself.
Gliscor and Politoed make excellent teammates for Tentacruel, the former because of its excellent synergy with Tentacruel, both in terms of typing and defensive stats; the latter because of its ability to bring unending rain to the battlefield. The rain allows Tentacruel to recover more HP from Rain Dish, as well as making sure Heatran doesn't start pulling out Sunny Specs Fire Blasts or Eruptions on you.