There's no universal "best" Companion. There's just too many variables between playing style, main character choices and desired goals for any one Companion build to be objectively better than all the others. There are broken Companion builds but as long as you avoid anything obviously problematic you should be okay.
Best advice? Pick a Companion you like and build him or her out in a manner that compliments your main character and playing style.
Some examples:
1) Gris as a group tank for a mage main character. Give her a shield, and group attacks and let her occupy the hordes while you blast away with strong spells.
2) Gris as a 2-H slayer with a focus on aggressive abilities, paired with a Mage. Best defense is a good offense followed by a fireball.
3) Hirge with healing as the only active skill paired with a 2-H warrior. She can hit hard, and can heal you main character as needed.
4) Hirge with a lot of active skills, including
Arbenos' Might. Paired with anyone. Sure, she'll end up healing fire elementals more often than not, but those pesky goblins won't know what hit them. And the undead will learn to stay dead.
5) Adaon, with high DPS build, paired with a 1-H Cleric. For long fights, the Cleric can heal. For short fights, Adaon stabs.
6) Adaon built as a thief, paired with anyone. Max out his trap expert skill, his secret finding skills, screw combat, he's there to find the loot. That frees up your main character to focus on combat abilities.
Those are just a few examples.