Okay, so a few people already replied to you, and their suggestions are probably the best partners, but I'm going to take my freedom to explain why those Pokemon are especially good partners :) Assuming you are running the standard Tentacruel moveset which consists of Scald | Clear Smog | Rapid Spin | Toxic Spikes, there are quite a few Pokemon who can switch in on you almost for free, and threaten you out, too.
The most prevelant thing is probably the Lati@s, as they are fairly common, and gets a free switch-in, only fearing a potential burn. Thus, Jirachi, Ferrothorn, Choice Scarf Tyranitar, and physically defensive Clefable make for great teammates. Tentacruel also struggles to take on Mega Sableye, Mega Sceptile, Mega Gallade, and Mega Metagross. In addition to this, strong physical attackers with Ground-type STAB, such as Diggersby, Landorus-T and Excadrill pose a threat to Tentacruel. Unaware Clefable is capable of taking on Mega Sableye, and specially defensive Clefable, and Mega Scizor can take on Mega Sceptile. Ferrothorn and Mega Scizor is great for taking on Mega Metagross, while Mega Sableye counters Mega Gallade. Chesnaught takes on Excadrill, and Diggersby to an extent, and can also pressure opposing teams with Spikes. Lastly, Landorus-T is probably the best overall partner to Tentacruel, alongside Clefable, as it checks a plethora of physical attackers, while being able to set Stealth Rock. n_n