It's hard to award a single Pokémon the title of "best" of any type, role, or stats, since each performs different roles with different strengths and weaknesses. However, in general terms, you'd probably be best choosing between Gengar, Amoonguss, and Scolipede.
-Gengar has phenomenal offensive presence. It has a solid movepool, amazing Speed and Special Attack stats, and great stats.
-Gengar typically fills the role of special sweeper. Whether it's used early-game to punch holes or late-game to clean up the opposing team, it'll be using special moves.
-Not a whole lot in Generation 6 can come in on Gengar easily. For the Pokémon that can do that, you'll have the rest of your team.
-Gengar has pretty great synergy with a lot of other strong Pokémon available in Generation 6.
-Amoonguss has access to Regenerator, phenomenal bulk, and a great defensive typing. It's very difficult to kill.
-Amoonguss comes with Spore, a great move to have that lulls the opponent to sleep 100% of the time if they don't have Overcoat, Safety Goggles, or aren't Grass-type.
-Amoonguss has access to a variety of support and recovery moves, like Clear Smog and Synthesis.
-Amoonguss has reliable STAB moves in Giga Drain and Sludge Bomb and can also effectively run Hidden Power Fire in order to aid in its reliability as a wall. It's able to fire back against things that come in against it.
-If you need a solid bulky Pokémon in your team, Amoonguss is the Poison-type Pokémon to go with. Make sure you have other Pokémon to help cover its weaknesses, though!
-Scolipede is a formidable Poison-type setup sweeper. It has great Speed and Attack, an amazing Ability in Speed Boost, strong STABs in Megahorn and Poison Jab, and decent coverage options.
-It takes a bit more support, but Scolipede is capable of running through teams once it has Attack and Speed Boosts stacked up. Just make sure things like Skarmory are out of the picture before you try to set up.
Hopefully this helped. It's always hard to pick out a Pokémon for a team without seeing the team itself, but I'm sure you know what you're looking for.