Re-Write of the Choice Specs set. There's no doubt that it was in need of it. I was sceptical of the set at first, but the surprise element and the respectable Speed convinced me in the end that it was an analysis worthy set:
http://www.smogon.com/dp/pokemon/tentacruel
[SET]
name: Choice Specs
move 1: Surf / Hydro Pump
move 2: Ice Beam
move 3: Sludge Bomb
move 4: Giga Drain / Hidden Power Grass / Hidden Power Electric
item: Choice Specs
ability: Liquid Ooze
nature: Timid
evs: 4 HP / 252 SpA / 252 Spe
[SET COMMENTS]
<p>With the Choice Specs, Tentacruel’s Special Attack stat reaches a respectable 388. Its natural bulk on the Special side allows it to switch in relatively easily. While many view Tentacruel as a supporting Pokemon, its relatively high base Speed can surprise opponents and can grab some useful KOes. Your choice of Surf or Hydro Pump depends on whether you prefer the power of Hydro Pump, or the accuracy of Surf, this is very much a taste decision. Ice Beam will allow you to hit Dragons hard, which can prove useful considering that you will be faster than a Dragonite, and will at worst Speed tie with any Flygon or Salamence who haven’t boosted their Speed via Dragon Dance or Choice Scarf. Note that Dragonite may also choose to boost its Speed. Sludge Bomb is a STAB option that will hit a lot of Pokemon for neutral damage, most notably Gyarados; offensive versions will be 2HKOed outright. Rapid Spin is an option that can be used over Sludge Bomb whilst scouting for Tentacruel's counters.</p>
<p>Giga Drain and Hidden Power Grass are predominantly there for Swampert, as neither will hit pure Water-types much harder than Sludge Bomb. Giga Drain will do 336 HP on average to the Standard MixPert, recovering over half of Tentacruel's health, but will fail to OHKO a full health MixPert. On the other hand, Hidden Power Grass will OHKO the mud fish 80% of the time with Stealth Rock up, but will deny Tentacruel the opportunity to leech half of that health back. If hitting Gyarados is a priority for you, Hidden Power Electric will OHKO even the bulkier versions after Stealth Rock.</p>
<p>The EVs are fairly self-explanatory; maximize Special Attack to hit as hard as possible, and maximize Speed to ensure at worst a Speed tie with other positive base 100 Speed Pokemon. Liquid Ooze is the preferred Ability here as it is unlikely that Tentacruel will receive any problematic stat drops, which would warrant the use of Clear Body, when running this set. Although it doesn't pack as much power as other Choice Specs Pokemon, this Tentacruel has the element of surprise; few expect Tentacruel to carry Sludge Bomb, Hidden Power, and Ice Beam all on one set. Even fewer expect it to have such high Speed. Use this element of surprise to your advantage. You could potentially use a Choice Scarf over Choice Specs on this set, which will allow you to revenge kill more of the OU metagame (most notably Gyarados and Salamence with one Dragon Dance), but the power loss is undesirable outside of those specific situations.</p>
http://www.smogon.com/dp/pokemon/tentacruel

[SET]
name: Choice Specs
move 1: Surf / Hydro Pump
move 2: Ice Beam
move 3: Sludge Bomb
move 4: Giga Drain / Hidden Power Grass / Hidden Power Electric
item: Choice Specs
ability: Liquid Ooze
nature: Timid
evs: 4 HP / 252 SpA / 252 Spe
[SET COMMENTS]
<p>With the Choice Specs, Tentacruel’s Special Attack stat reaches a respectable 388. Its natural bulk on the Special side allows it to switch in relatively easily. While many view Tentacruel as a supporting Pokemon, its relatively high base Speed can surprise opponents and can grab some useful KOes. Your choice of Surf or Hydro Pump depends on whether you prefer the power of Hydro Pump, or the accuracy of Surf, this is very much a taste decision. Ice Beam will allow you to hit Dragons hard, which can prove useful considering that you will be faster than a Dragonite, and will at worst Speed tie with any Flygon or Salamence who haven’t boosted their Speed via Dragon Dance or Choice Scarf. Note that Dragonite may also choose to boost its Speed. Sludge Bomb is a STAB option that will hit a lot of Pokemon for neutral damage, most notably Gyarados; offensive versions will be 2HKOed outright. Rapid Spin is an option that can be used over Sludge Bomb whilst scouting for Tentacruel's counters.</p>
<p>Giga Drain and Hidden Power Grass are predominantly there for Swampert, as neither will hit pure Water-types much harder than Sludge Bomb. Giga Drain will do 336 HP on average to the Standard MixPert, recovering over half of Tentacruel's health, but will fail to OHKO a full health MixPert. On the other hand, Hidden Power Grass will OHKO the mud fish 80% of the time with Stealth Rock up, but will deny Tentacruel the opportunity to leech half of that health back. If hitting Gyarados is a priority for you, Hidden Power Electric will OHKO even the bulkier versions after Stealth Rock.</p>
<p>The EVs are fairly self-explanatory; maximize Special Attack to hit as hard as possible, and maximize Speed to ensure at worst a Speed tie with other positive base 100 Speed Pokemon. Liquid Ooze is the preferred Ability here as it is unlikely that Tentacruel will receive any problematic stat drops, which would warrant the use of Clear Body, when running this set. Although it doesn't pack as much power as other Choice Specs Pokemon, this Tentacruel has the element of surprise; few expect Tentacruel to carry Sludge Bomb, Hidden Power, and Ice Beam all on one set. Even fewer expect it to have such high Speed. Use this element of surprise to your advantage. You could potentially use a Choice Scarf over Choice Specs on this set, which will allow you to revenge kill more of the OU metagame (most notably Gyarados and Salamence with one Dragon Dance), but the power loss is undesirable outside of those specific situations.</p>